The URI http://www.w3.org/2006/02/soap12/abstractions is used to identify certain concepts and abstractions which are formally defined in the SOAP Version 1.2 specification.
For more information, please see the the home page of the XML Protocol Working Group, who produced the SOAP Version 1.2 specification.
Individual concepts and abstractions are itemized below.
http://www.w3.org/2006/02/soap12/abstractions#classOfMEPs is a URI that identifies the class of all possible SOAP Message Exchange Patterns.
The term Message Exchange Pattern is defined in the SOAP 1.2 Recommendation (latest version). Note that some MEPs are defined in the SOAP Recommendation, but others may be defined too. The URI http://www.w3.org/2006/02/soap12/abstractions#classOfMEPs refers to the class of all such MEPs, whether defined by the W3C or by others.