3GPP 24.501 v20.0.0 — the document's own text
8.2.7.40 Extended rejected NSSAI
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
If the UE supports Extended rejected NSSAI, the network may include this IE to inform the UE of one or more S-NSSAIs that were included in the requested NSSAI in the REGISTRATION REQUEST message but were rejected by the network.