ReferenceDescription is defined in the spec as a core Data Type. It is assigned DataType NodeId i=518 but is not in Opc.Ua.Nodeset2.xml. Probably because of this it does not appear in in the type system under the Types\DataTypes node. This prevents an application from creating a custom structure that includes a field of that type, or a custom structure derived from that type.

For completeness this should be included.

Possible use cases:
Server creates a structure which contains an array of RDs so clients don't have to manually browse a subset of the address space.
Server implements a custom query method with an input argument filter that is a DSL, which returns one or more RDs.
Server derives a custom structured type from RD that adds additional information, such as a count of the target Node's forward references, so that if the target does not have any fwd refs the client does not need to browse it.