OPC UA is a set of layered specifications broken into multiple parts. The base and functional parts of OPC UA are described in abstract terms. A separate implementation part defines the mapping to existing protocol and security technologies on which interoperable 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.
To support different application domains with different requirements, OPC UA also allows multiple mappings for different protocols and encodings.
Each mapping consists of three functional layers: Data Encoding, Security Protocol and Transport Protocol. Different mappings are combined together to create Transport Capabilities.
Note - Transport Mappings are specified in OPC UA Part 6