When an UE is switched on, it attempts to make contact with a PLMN. The selection of the PLMN could be either automatically or manually.

Automatic mode: Utilizes a list of PLMN to be selected in priority order.

Manual mode: The mobile station indicates available PLMN to the user. Upon the user selection the UE fetches service from the new PLMN.

Normally the UE operates on its home PLMN. At home PLMN, the MCC (Mobile
Country Code) and MNC (Mobile Network Code) of the PLMN identity match the
MCC and MNC of the IMSI (International Mobile Subscriber Identity) stored on the USIM (UMTS Subscriber Identity Module).

LTE PLMN

If the UE loses its present PLMN coverage, a new available PLMN is selected
automatically or manually. If necessary, the UE will do “reselection” by searching a suitable cell on a different PLMN.