Details can be found at : 3GPP TS 23.228 V10.5.0 (2011-06) page201 under clauses (E.2.1.0,E.2.1.1,E.2.1.2)
Requirements for IM CN subsystem signalling flag :-
The IM CN Subsystem Signalling flag is used to indicate the dedicated signalling PDP context for IMS signalling. When this flag should be active and which cases should be enabled an used