The DataType Model is used to define simple and structured data types. Data types are used to describe the structure of the Value Attribute of Variables and their VariableTypes. Therefore each Variable and VariableType is pointing with its DataType Attribute to a Node of the DataType NodeClass as shown in Figure 10.
In many cases, the NodeId of the DataType Node – the DataTypeId – will be well-known to Clients and Servers. Clause 8 defines DataTypes and OPC 10000-6 defines their DataTypeIds. In addition, other organizations may define DataTypes that are well-known in the industry. Well-known DataTypeIds provide for commonality across OPC UA Servers and allow Clients to interpret values without having to read the type description from the Server. Therefore, Servers may use well-known DataTypeIds without representing the corresponding DataType Nodes in their AddressSpaces.
In other cases, DataTypes and their corresponding DataTypeIds may be vendor-defined. Servers should attempt to expose the DataType Nodes and the information about the structure of those DataTypes for Clients to read, although this information might not always be available to the Server.
Figure 11 illustrates the Nodes used in the AddressSpace to describe the structure of a DataType. The DataType points to an Object of type DataTypeEncodingType. Each DataType can have several DataTypeEncoding, for example “Default”, “UA Binary” and “XML” encoding. Services in OPC 10000-4 allow Clients to request an encoding or choosing the “Default” encoding. Each DataTypeEncoding is used by exactly one DataType, that is, it is not permitted for two DataTypes to point to the same DataTypeEncoding.
Since the NodeId of the DataTypeEncoding will be used in some Mappings to identify the DataType and it’s encoding as defined in OPC 10000-6, those NodeIds may also be well-known for well-known DataTypeIds.