LTE: what is GU GROUP ID in lte, when it is used and what is the significance of it?

Question

I gone through the RFC4006 but didnt get the context of below paragraph

   Certain applications require multiple credit-control sub-sessions.
   These applications would send messages with a constant Session-Id
   AVP, but with a different CC-Sub-Session-Id AVP.  If several credit
   sub-sessions will be used, all sub-sessions MUST be closed separately
   before the main session is closed so that units per sub-session may
   be reported.  The absence of this AVP implies that no sub-sessions
   are in use.

But here my question is
1. What could be the types of Multiple credit-ontrol Sub-session?
2. How can we include different cc-sub-session-ID for each sub-session, please give me an example with message structure using CCR message.

Answer 1

GU Group ID is the globally unique group id corresponding to a pool area.
GU Group ID = PLMN ID + MME Group ID

Mostly we can see this at X2 based procedures like given below.
1) X2 Setup Request
2) X2 Setup Response
3) eNB configuration update

Reference Spec:-36.423

Comments

The Group ID may be used in the X2 setup, but it is more specifically used as an identifier of the MME so the establishment of the S1 would be the correct link establishment.
It is used to identify which MME pool the MME is a part of, and this links to the UE's Globally unique ID (GUTI) which includes the group ID.
GUTI = MCC + MNC + MMEGI + MMEC + M-TMSI
Now in English
Globally Unique Temporary Identifier (UE) = Mobile Contry Code (MCC) + the Mobile Network Code (MNC)  (MCC+MNC=PLMN ID) + MME Group ID + MME Code (MMEGI + MMEC = MMEI  and PLMN ID + MMEI = Globally Unique MME Identity (GUMMEI) + MME-Moble Temporary Identifier (M-TMSI).

I hope you enjoyed the alphabet soup.