OPC UA provides a framework that can be used to represent complex information as Objects in an AddressSpace which can be accessed with standard services. These Objects consist of Nodes connected by References. Different classes of Nodes convey different semantics. For example, a Variable Node represents a value that can be read or written. The Variable Node has an associated DataType that can define the actual value, such as a string, float, structure etc. It can also describe the Variable value as a variant. A Method Node represents a Function that can be called. Every Node has a number of Attributes, including a unique identifier called a NodeId and non-localized name called a BrowseName. An Object representing a “Reservation” is shown in Figure 6.

Figure 6 – A basic Object in an OPC UA Address Space

Object and Variable Nodes represent instances and always reference a TypeDefinition (ObjectType or VariableType) Node which describes their semantics and structure. Figure 7 illustrates the relationship between an instance and its TypeDefinition.

Type Nodes are templates that define all the children that can be present in an instance of the type. In the example in Figure 7 the “PersonType” ObjectType defines two children: First Name and Last Name. All instances of “PersonType” are expected to have the same children with the same BrowseNames. Within a type, the BrowseNames uniquely identify the children. This means Client applications can be designed to search for children based on the BrowseNames from the type instead of NodeIds. This eliminates the need for manual reconfiguration of systems if a Client uses types that are implemented on multiple Servers.

OPC UA also supports the concept of subtyping. This allows a modeller to take an existing type and extend it. Rules regarding subtyping are defined in OPC 10000-3, but in general they allow the extension of a given type or the restriction of a DataType. For example, the modeller may decide that the existing ObjectType needs an additional Variable in some cases. The modeller can create a subtype of the ObjectType and add the Variable. A Client that is expecting the parent type can treat the new type as if it were of the parent type. Regarding DataTypes, subtypes can only restrict. If a Variable is defined to have a numeric value, a subtype could restrict it to a float.

Figure 7 – The relationship between Type Definitions and Instances

References allow Nodes to be connected in ways that describe their relationships. All References have a ReferenceType that specifies the semantics of the relationship. References can be hierarchical or non-hierarchical. Hierarchical references are used to create the structure of Objects and Variables, non-hierarchical references are used to create arbitrary associations. Applications can define their own ReferenceType by creating subtypes of an existing ReferenceType. Subtypes inherit the semantics of the parent but may add additional restrictions. Figure 8 depicts several References connecting different Objects.

Figure 8 – Examples of References between Objects

The figures above use a notation that was developed for the OPC UA specification. This notation is summarized in Figure 9. UML representations can also be used; however, the OPC UA notation is less ambiguous because there is a direct mapping from the elements in the figures to Nodes in the AddressSpace of an OPC UA Server.

Figure 9 – The OPC UA Information Model notation

A complete description of the different types of Nodes and References can be found in OPC 10000-3 and the base structure is described in OPC 10000-5.

The OPC UA specification defines a very wide range of functionalities in its basic information model. It is not required that all Clients or Servers support all functionalities in the OPC UA specifications. OPC UA includes the concept of Profiles, which segment the functionality into testable certifiable units. This allows the definition of functional subsets (that are expected to be implemented) within a companion specification. Profiles do not restrict functionality, but they generate requirements for a minimum set of functionalities (see OPC 10000-7).

OPC UA allows information from many different sources to be combined into a single coherent AddressSpace. Namespaces make this possible by eliminating naming and ID conflicts between information from different sources. Each Namespace in OPC UA has a globally unique string called a NamespaceUri which identifies a naming authority, and a locally unique integer called a NamespaceIndex which is an index into the Server’s table of NamespaceUris. The NamespaceIndex is unique only within the context of a Session between an OPC UA Client and an OPC UA Server - the NamespaceIndex  can change between Sessions and still identify the same item even though the NamespaceUri’s location in the table has changed. The Services defined for OPC UA use the NamespaceIndex to specify the Namespace for qualified values.

There are two types of structured values in OPC UA that are qualified with NamespaceIndexes: NodeIds and QualifiedNames. NodeIds are locally unique (and sometimes globally unique) identifiers for Nodes. The same globally unique NodeId  can be used as the identifier in a Node in many Servers – the Node's instance data may vary but its semantic meaning is the same regardless of the Server it appears in. This means Clients can have built-in knowledge of what the data means in these Nodes. OPC UA Information Models generally define globally unique NodeIds for the TypeDefinitions defined by the Information Model.

QualifiedNames are non-localized names qualified with a Namespace. They are used for the BrowseNames of Nodes and allow the same names to be used by different information models without conflict. TypeDefinitions are not allowed to have children with duplicate BrowseNames; however, instances do not have that restriction.

An OPC UA companion specification for an industry-specific vertical market describes an Information Model by defining ObjectTypes, VariableTypes, DataTypes and ReferenceTypes that represent the concepts used in the vertical market, as well as potentially well-defined Objects as entry points into the AddressSpace.