3GPP 23.501 v20.2.0 — the document's own text

5.9.8 Generic Public Subscription Identifier

Taught in 7. Every name the system uses (The 5G system architecture, in depth), 4. Knowing who you are and where you are (The 5G system architecture, overview).

Generic Public Subscription Identifier (GPSI) is needed for addressing a 3GPP subscription in different data networks outside of the 3GPP system. The 3GPP system stores within the subscription data the association between the GPSI and the corresponding SUPI.

GPSIs are public identifiers used both inside and outside of the 3GPP system.

The GPSI is either an MSISDN or an External Identifier, see TS 23.003 [19]. If MSISDN is included in the subscription data, it shall be possible that the same MSISDN value is supported in both 5GS and EPS.

NOTE: There is no implied 1-to-1 relationship between GPSI and SUPI.