Registered PLMN is the PLMN (Public Land Mobile Network) on which certain LR (Location Registration) outcomes have occurred.
1)UE power ON happens, USIM module is initialized and USIM module send USIM active Indication or USIM not present message to NAS EMM module
2)On receiving this, EMM goes and read PLMN specific information from USIM like RPLMN ID, HPLMN ID list, EHPLMN ID list, VPLMN ID list. (not sure whether this is a request response pattern or reading from some file)
3)EMM gives RRC RPLMN ID if available, RRC goes on and check whether it can camp on that PLMN.
4)If it is sucessful, RRC indicates sucess and gives back the current TAI, acess baring info etc....
5)Immediately NAS send Attach Request and attach type could be IMSI attach or combined attach.
6)If UE could not camp on provided PLMN, NAS gives the next PLMN ID in order of priority. If UE cannot camp on any RPLMN or HPLMN or EHPLMN or VPLMN id, UE camps on the strongest cell and service type will be emergency in that case.
