OPC UA is a set of layered specifications broken into multiple parts. The Base and Functional Parts are described in abstract terms. The implementation parts define the mapping to existing protocol and security technologies on which software can be built.
This layering is purposely intended to isolate the architectural framework from the inevitable changes in the technology used to implement it. So as new technologies arrive, OPC UA will be able to advance with them. It will be possible to add new over-the-wire Protocols, and to add new Security and Encryption technologies without changing the functional elements of OPC UA.
OPC UA is applied to different application domains with different requirements. Therefore it supports multiple mappings for different protocols and encodings.
The mappings consist of three functional layers: Data Encodings, Security Protocols and Transport Protocols. Different mappings are combined together to create Transport Capabilities.
Note - Transport Mappings are specified in OPC UA Part 6