3GPP 24.501 v20.0.0 — the document's own text
8.2.6.28 Requested mapped NSSAI
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
This IE shall be included by the UE when the UE has a PDN connection or a PDU session to transfer to visited PLMN associated only with an HPLMN S-NSSAI as specified in clause 5.5.1.3.2.