I would like to add few points here. LTE core network has so many nodes likes MME, SGW, PGW/PCEF, PCRF, AF , SPR/HSS . Here when we talk about network initiated it means, Application function is initiating. AF triggers control message to PCRF for an UE and then PCRF sends diameter RAR message to PCEF. PCEF/PGW sits together so once PCEF receives message from PCRF, PGW sends create initiates "create bear request" to SGW and SGW forwards this message to MME. Please go through 3GPP 29.212 and 29.214 specifications. There is a "Subscription-Id " AVP which is used to carry UE subscription related informations like IMSI, MSISDN number in both Rx (interface between PCRF<->AF) and Gx (interface between PRCF and PCEF).