DIAMETER based interfaces in EPC, UMTS and IMS

Recently I've seen question regarding on which interface in LTE or UMTS the DIAMETER protocol is used.

Basically DIAMETER is an Authentication, Authorization & Accounting (AAA)  protocol. That is why any of you will see DIAMETER used for these functions.

                    S6a - Authentication, more in TS 29.272
                    Gy - Prepaid charging, more in TS 23.203, TS 32.299;
                    Gz - Postpaid charging;
                    Gx - QoS/Policy, more in TS 29.211, TS 29.212;
                    Rf - Charging, more in TS 32.299;
                    Ro - Charging, more in TS 32.299;
                    Rx - QoS/Policy, more in TS 29.214;
                    S6d - Authentication;
                    S9 - QoS/Policy;
                    Sh - Subscriber Profile;
                    Cx - Subscriber Profile;
                    e2 - Location.
But also few others specific to IMS


                    Dh - used by AS to find the HSS holding the User Profile in multi-HSS environment;
                    Dx - used by I-CSCF or S-CSCF to find a correct HSS in a multi-HSS environment;
                    Gq - to exchange policy decisions-related information between P-CSCF and PDF;

If you are working on the UE side then the DIAMETER is transparent. Although you could see some DIAMETER protocol reference while working with VoLTE over IMS.

Source(s):
LinkedIn, 
my own experience
3GPP Specyfications