EMA C++ RDM Usage Guide : 4 Source Directory Domain : 4.4 Special Semantics : 4.4.2 Service IDs
 
4.4.2 Service IDs
Most Domain Model messages can be associated with a service (although Login and Directory typically are not). For better bandwidth utilization, the RSSL transport optimizes the service name into a two byte service ID. The ServiceId is only unique within a single channel.