3GPP 24.501 v20.0.0 — the document's own text
8.2.25.4 Selected EPS NAS security algorithms
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 if the AMF supports N26 interface, the UE set the S1 mode bit to "S1 mode supported" in the 5GMM capability IE of the REGISTRATION REQUEST message, and the AMF needs to provide the selected EPS NAS security algorithms to the UE.