Different kinds of DataTypes are handled differently regarding their encoding and according to whether this encoding is represented in the AddressSpace.
Built-in DataTypes are a fixed set of DataTypes (see OPC 10000-6 for a complete list of Built-in DataTypes). They have no encodings visible in the AddressSpace since the encoding should be known to all OPC UA products. Examples of Built-in DataTypes are Int32 (see 8.26) and Double (see 8.12).
Simple DataTypes are subtypes of the Built-in DataTypes. They are handled on the wire like the Built-in DataType, i.e. they cannot be distinguished on the wire from their Built-in supertypes. Since they are handled like Built-in DataTypes regarding the encoding they cannot have encodings defined in the AddressSpace. Clients can read the DataType Attribute of a Variable or VariableType to identify the Simple DataType of the Value Attribute. An example of a Simple DataType is Duration. It is handled on the wire as a Double but the Client can read the DataType Attribute and thus interpret the value as defined by Duration (see 8.13).
Structured DataTypes are DataTypes that represent structured data and are not defined as Built-in DataTypes. Structured DataTypes inherit directly or indirectly from the DataType Structure defined in 8.33. Structured DataTypes may have several encodings and the encodings are exposed in the AddressSpace. How the encoding of Structured DataTypes is handled on the wire is defined in OPC 10000-6. The encoding of the Structured DataType is transmitted with each value, thus Clients are aware of the DataType without reading the DataType Attribute. The encoding has to be transmitted so the Client is able to interpret the data. An example of a Structured DataType is Argument (see 8.6).
Enumeration DataTypes are DataTypes that represent discrete sets of named values. Enumerations are always encoded as Int32 on the wire as defined in OPC 10000-6. Enumeration DataTypes inherit directly or indirectly from the DataType Enumeration defined in 8.14. Enumerations have no encodings exposed in the AddressSpace. To expose the human-readable representation of an enumerated value the DataType Node may have the EnumStrings Property that contains an array of LocalizedText. The Integer representation of the enumeration value points to a position within that array. EnumValues Property can be used instead of the EnumStrings to support integer representation of enumerations that are not zero-based or have gaps. It contains an array of a Structured DataType containing the integer representation as well as the human-readable representation. An example of an enumeration DataType containing a sparse list of Integers is NodeClass which is defined in 8.30.
In addition to the DataTypes described above, abstract DataTypes are also supported, which do not have any encodings and cannot be exchanged on the wire. Variables and VariableTypes use abstract DataTypes to indicate that their Value may be any one of the subtypes of the abstract DataType. An example of an abstract DataType is Integer which is defined in 8.24.