Any Instanceof a TypeDefinitionNodewill be the root of a hierarchy which mirrors the InstanceDeclarationHierarchyfor the TypeDefinitionNode.Each Nodein the hierarchy of the Instance will have a BrowsePathwhich may be the same as the BrowsePathfor one of the InstanceDeclarationsin the hierarchy of the TypeDefinitionNode. The InstanceDeclarationwith the same BrowsePathis called the InstanceDeclarationfor the Node. If a Nodehas an InstanceDeclarationthen it shall have the same BrowseNameand NodeClassas the InstanceDeclarationand, in cases of Variablesand Objects, the same TypeDefinitionNodeor a subtype of it.

Instances may reference several Nodeswith the same BrowsePath. Clientsthat need to distinguish between the Nodesbased on the InstanceDeclarationHierarchyand the Nodesthat are not based on the InstanceDeclarationHierarchycan accomplish this using the TranslateBrowsePathsToNodeIds service defined in OPC 10000-4.

Instances inherit the initial values for the Attributesthat they have in common with the TypeDefinitionNodefrom which they are instantiated, with the exceptions of the NodeClassand NodeId.

When a Servercreates an instance of a TypeDefinitionNodeit shall create the same hierarchy of Nodes beneath the new Objector Variable depending on the ModellingRuleof each InstanceDeclaration. Standard ModellingRulesare defined in The Nodes within the newly created hierarchy may be copies of the InstanceDeclarations, the InstanceDeclarationitself or another Nodein the AddressSpacethat has the same TypeDefinitionNodeand BrowseName. If new copies are created, then the Attributevalues of the InstanceDeclarationsare used as the initial values. An instance shall not be a SourceNodeof a hierarchical Referencethat has the same BrowsePathas an InstanceDeclarationof its TypeDefinitionwhich has an optional or mandatory ModellingRuleexcept for the one based on the InstanceDeclaration.

Figure 22provides a simple example of a TypeDefinitionNodeand an Instance. Nodesreferenced by the TypeDefinitionNodewithout a ModellingRuledo not appear in the instance. Instancesmay have children with duplicate BrowseNames; however, only one of those children will correspond to the InstanceDeclaration.


Figure 22– An Instance and its TypeDefinitionNode

It is up to the Serverto decide which InstanceDeclarationsappear in any single instance. In some cases, the Serverwill not define the entire instance and will provide remote references to Nodesin another Server. The ModellingRulesdescribed in Serversto indicate that some Nodesare always present; however, the Clientshall be prepared for the case where the Nodeexists in a different Server.

A Clientcan use the information of TypeDefinitionNodesto access Nodes which are in the hierarchy of the instance. It shall pass the NodeIdof the instance and the BrowsePathof the child Nodes based on the TypeDefinitionNodeto the TranslateBrowsePathsToNodeIdsservice (see OPC 10000-4). This Servicereturns the NodeIdfor each of the child Nodes. If a child Nodeexists then the BrowseNameand NodeClassshall match the InstanceDeclaration. In the case of Objectsor Variables,also the TypeDefinitionNodeshall either match or be a subtype of the original TypeDefinitionNode.

Objectsand Variablesmay change their Attributevalues after being created. Special rules apply for some Attributesas defined in 6.2.6.

Additional Referencesmay be added to the Nodes, and Referencesmay be deleted as long as the ModellingRulesdefined on the InstanceDeclarationsof the TypeDefinitionNodeare still fulfilled.

For Variablesand Objectsthe HasTypeDefinition Referenceshall always point to the same TypeDefinitionNodeas the InstanceDeclarationor a subtype of it.

If two InstanceDeclarationsof the fully-inherited InstanceDeclarationHierarchyhave been connected directly with several References, all those Referencesshall connect the same Nodes. An example is given in Figure 23. The instances A1 and A2 are allowed since B1 references the same Nodewith both References, whereas A3 is not allowed since two different Nodesare referenced. Note that this restriction only applies for directly connected Nodes. For example, A2 references a C1 directly and a different C1 via B1.


Figure 23– Example for several References between InstanceDeclarations

For a definition of ModellingRules, see Other parts of this series of standards may define additional ModellingRules. ModellingRulesare an extendable concept in OPC UA; therefore, vendors may define their own ModellingRules.

Note that the ModellingRulesdefined in this standard do not define how to deal with NonHierarchical Referencesbetween InstanceDeclarations, i.e. it is Server-specific if those Referencesexist in an instance hierarchy or not. Other ModellingRulesmay define behaviour for NonHierarchical Referencesbetween InstanceDeclarationas well.

ModellingRulesare represented in the AddressSpaceas Objectsof the ObjectType ModellingRuleType. There are some Propertiesdefining common semantic of ModellingRules.This edition of this standard only specifies one Propertyfor ModellingRules. Future editions may define additional Propertiesfor ModellingRules. OPC 10000-5specifies the representation of the ModellingRule Objects, their Propertiesand their type in the AddressSpace.

Subclause how the ModellingRulemay be changed when instantiating InstanceDeclarationswith respect to the Properties. Subclause how the ModellingRulemay be changed when overriding InstanceDeclarationsin subtypes with respect to the Properties.

It is allowed that subtypes override ModellingRules on their InstanceDeclarations. As a general rule for subtyping, constraints shall only be tightened, not loosened. Therefore, it is not allowed to specify on the supertype that an instance shall exist with the ModellingRule Mandatory and on the subtype make this ModellingRule Optional. Table 20specifies the allowed changes on the Properties when overriding the ModellingRules in the subtype.

Table 20– Rule for ModellingRules Properties when Subtyping

ModellingRule on supertype

ModellingRule on subtype




Mandatory or Optional




MandatoryPlaceholder or OptionalPlaceholder

There are two different use cases when creating an instance ‘A’ based on a TypeDefinitionNode‘A_Type’. Either ‘A’ is used as normal instance or it is used as an InstanceDeclarationof another TypeDefinitionNode.

In the first case, it is not required that newly created or referenced instances based on InstanceDeclarationshave a ModellingRule, however, it is allowed that they have any ModellingRuleindependent of the ModellingRuleof their InstanceDeclaration.

In Figure 24an example is given. The instances A1, A2, and A3 are all valid instances of Type_A, although B of A1 has no ModellingRuleand B of A3 has a different ModellingRulethan B of Type_A.


Figure 24– Example on changing instances based on InstanceDeclarations

In the second case, all instances that are referenced directly or indirectly from ‘A’ based on InstanceDeclarationsof ‘A_Type’ initially maintain the same ModellingRuleas their InstanceDeclarations.The ModellingRulesmay be updated; the allowed changes to the ModellingRulesof these Nodes are the same as those defined for subtyping in

In Figure 25an example of such a scenario is given. Type_B uses an InstanceDeclarationbased on Type_A (upper part of the Figure). Later on the ModellingRuleof the InstanceDeclaration A1 is changed (lower part of the Figure). A1 has become the NamingRuleof Mandatory(changed from Optional).


Figure 25– Example on changing InstanceDeclarations based on an InstanceDeclaration

An InstanceDeclarationmarked with the ModellingRule Mandatorymeans that for each existing BrowsePathon the instance a similar Nodeshall exist, but it is not defined whether a new Nodeis created or an existing Nodeis referenced.

For example, the TypeDefinitionNodeof a functional block “AI_BLK_TYPE” will have a setpoint “SP1”. An instance of this type “AI_BLK_1” will have a newly-created setpoint “SP1” as a similar Node to the InstanceDeclarationSP1. Figure 26illustrates the example.


Figure 26– Use of the Standard ModellingRule Mandatory

In complex example combining the Mandatoryand Optional ModellingRulesis given.

An InstanceDeclarationmarked with the ModellingRule Optionalmeans that for each existing BrowsePathon the instance a similar Nodemay exist, but it is not defined whether a new Nodeis created or an existing Nodeis referenced.

In Figure 27an example using the ModellingRules Optionaland Mandatoryis shown. The example contains an ObjectTypeType_A and all valid combinations of instances named A1 to A13. Note that if the optional B is provided, the mandatory E has to be provided as well, otherwise not. F is referenced by C and D. On the instance, this can be the same Nodeor two different Nodeswith the same BrowseName(similar Nodeto InstanceDeclarationF). Not considered in the example is if the instances have ModellingRulesor not. It is assumed that each F is similar to the InstanceDeclarationF, etc.

If there would be a NonHierarchical Referencebetween E and F in the InstanceDeclarationHierarchy, it is not specified if it occurs in the instance hierarchy or not. In the case of A10, there could be a reference from E to one F but not to the other F, or to both or none of them.


Figure 27– Example using the Standard ModellingRules Optional and Mandatory

The ExposesItsArray ModellingRuleexposes a special semantic on VariableTypeshaving a single- or multidimensional array as the data type. It indicates that each value of the array will also be exposed as a Variablein the AddressSpace.

The ExposesItsArray ModellingRulecan only be applied on InstanceDeclarationsof NodeClass Variablethat are part of a VariableTypehaving a single- or multidimensional array as its data type.

The VariableA having this ModellingRuleshall be referenced by a forward hierarchical Referencefrom a VariableType B. B shall have a ValueRankvalue that is equal to or larger than zero. A should have a data type that reflects at least parts of the data that is managed in the array of B. Each instance of B shall reference one instance of A for each of its array elements. The used Referenceshall be of the same type as the hierarchical Referencethat connects B with A or a subtype of it. If there are more than one forward hierarchical Referencesbetween A and B, then all instances based on B shall be referenced with all those References.

Figure 28gives an example. A is an instance of Type_A having two entries in its value array. Therefore it references two instances of the same type as the InstanceDeclarationArrayExpose. The BrowseNamesof those instances are not defined by the ModellingRule. In general, it is not possible to get a Variablerepresenting a specific entry in the array (e.g. the second). Clientswill typically either get the array or access the Variablesdirectly, so there is no need to provide that information.


Figure 28– Example on using ExposesItsArray

It is allowed to reference A by other InstanceDeclarationsas well. Those Referenceshave to be reflected on each instance based on A.

Figure 29gives an example. The PropertyEUUnit is referenced by ArrayExpose and therefore each instance based on ArrayExpose references the instance based on the InstanceDeclarationEUUnit.


Figure 29– Complex example on using ExposesItsArray

For Objectand Variablethe intention of the ModellingRule OptionalPlaceholderis to expose the information that a complex TypeDefinitionexpects from instances of the TypeDefinitionto add instances with specific Referenceswithout defining BrowseNamesfor the instances. For example, a Device might have a Folder for DeviceParameters, and the DeviceParameters should be connected with a HasComponent Reference. However, the names of the DeviceParameters are specific to the instances. The example is shown in Figure 30, where an instance Device A adds two DeviceParameters in the Folder.


Figure 30– Example using OptionalPlaceholder with an Object and Variable

The ModellingRule OptionalPlaceholderadds no additional constraints on instances of the TypeDefinition. It just provides useful information when exposing a TypeDefinition. When the InstanceDeclarationis complex, i.e. it references other InstanceDeclarationswith hierarchicalReferences, these InstanceDeclarationsare not further considered for instantiating the TypeDefinition.

It is recommended that the BrowseNameand the DisplayNameof InstanceDeclarationshaving the OptionalPlaceholder ModellingRule shouldbe enclosed within angle brackets.

When overriding the InstanceDeclaration, the ModellingRuleshall remain OptionalPlaceholder.

For Methods,the ModellingRule OptionalPlaceholder is used to define the BrowseNamewhere subtypes and instances provide more information. The Method definition with the OptionalPlaceholder only defines the BrowseName. Aninstance or subtype defines the InputArgumentsand OutputArguments. A subtype shall also change the ModellingRuleto Optionalor Mandatory. The Methodis optional for instances.For example, a Device might have a Methodto perform calibration however the specific arguments for the Methoddepend on the instance of the Device. In this example Device A does not implement the Method, Device B implements the Methodwith no arguments and Device C implements the Methodaccepting a mode argument to select how the calibration is to be performed. The example is shown in Figure 31.


Figure 31– Example using OptionalPlaceholder with a Method

For Objectand Variablethe ModellingRule MandatoryPlaceholderhas a similar intention as the ModellingRule OptionalPlaceholder. It exposes the information that a TypeDefinitionexpects of instances of the TypeDefinitionto add instances defined by the InstanceDeclaration. However, MandatoryPlaceholderrequires that at least one of those instances shall exist.

For example, when the DeviceType requires that at least one DeviceParameter shall exist without specifying the BrowseNamefor it, it uses MandatoryPlaceholderas shown in Figure 32. Device A is a valid instance as it has the required DeviceParameter. Device B is not valid as it uses the wrong ReferenceTypeto reference a DeviceParameter (Organizesinstead of HasComponent) and Device C is not valid because it does not provide a DeviceParameter at all.


Figure 32– Example on using MandatoryPlaceholder for Object and Variable

The ModellingRule MandatoryPlaceholderrequires that each instance provides at least one instance with the TypeDefinitionof the InstanceDeclarationor a subtype, and is referenced with the same ReferenceTypeor a subtype as the InstanceDeclaration. It does not require a specific BrowseNameand thus cannot be used for the TranslateBrowsePathsToNodeIds Service(see OPC 10000-4).

When the InstanceDeclarationis complex, i.e. it references other InstanceDeclarationswith hierarchicalReferences, these InstanceDeclarationsare not further considered for instantiating the TypeDefinition.

It is recommended that the BrowseNameand the DisplayNameof InstanceDeclarationshaving the MandatoryPlaceholder ModellingRule should be enclosed within angle brackets.

When overriding the InstanceDeclaration, the ModellingRuleshall remain MandatoryPlaceholder.

For Methods,the ModellingRule MandatoryPlaceholder is used to define the BrowseNamewhere subtypes and instances provide more information. The Method definition with the MandatoryPlaceholder only defines the BrowseName. Aninstance or subtype defines the InputArgumentsand OutputArguments. A subtype shall also change the ModellingRuleto Mandatory. The Methodis mandatory for instances.