How it Works
The operator PLMNs configured under PLMN list includes all UE PLMNs and serving PLMNs.
On receiving the Create Request from 4G or 5G RAT (SmContextCreate or Create Session Request), the SMF extracts UE PLMN from SUPI. SMF compares the UE PLMN and serving PLMN with the configured PLMN list and populates the PLMN in external messaging. The SMF determines the roaming status of subscribers based on the HPLMN values.
If the UE PLMN and the serving PLMN both belong to the PLMN list that is configured in SMF, then it is a home subscriber. If the UE PLMN does not belong to the configured PLMN list and the serving PLMN belongs to the configured PLMN list, then it is a visitor. If the UE PLMN belongs to the configured PLMN list and the serving PLMN does not belong to the configured PLMN list, then it is a roamer.