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.
my own experience