3GPP C  Application 

The C interface is located between the HLR and a GMSC or a SMS-G. When a call originates from outside the network, i.e. from the PSTN or another mobile network it has to pass through the gateway so that routing information required to complete the call may be gained. The protocol used for communication is MAP/C, the letter C indicating that the protocol is used for the C interface. In addition to this, the MSC may optionally forward billing information to the HLR after the call is completed and cleared down.

Reference:  3GPP TS29.002   View