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.

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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.