3GPP 24.501 v20.0.0 — the document's own text
8.2.6.27 UE radio capability ID
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 UE is not in NB-N1 mode, the UE supports RACS and the UE needs to signal a UE radio capability ID to the network.