IMSI
International Mobile Subscriber Identity: a unique identifier stored on the SIM/USIM that identifies a mobile subscriber, composed of MCC, MNC, and MSIN.
The IMSI is the number that actually identifies a subscriber to the network, distinct from the phone number people dial (that's the MSISDN). It's stored on the USIM and is built from three parts: the Mobile Country Code, the Mobile Network Code, and the MSIN that uniquely identifies the subscriber within that operator. The MCC+MNC pair tells any network which home operator a subscriber belongs to, which is what makes roaming routing work.
In 4G the IMSI was occasionally sent over the air in the clear during attach, and that's precisely what IMSI catchers abused to track devices. 5G closes the hole by never transmitting it openly — the permanent identity becomes the SUPI and it travels concealed as the SUCI. The IMSI itself hasn't disappeared; it's still the underlying value on the SIM and in the home subscriber database, and temporary identities (GUTI/TMSI) continue to stand in for it during normal signalling to avoid repeated exposure.
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