3GPP 24.501 v20.0.0 — the document's own text
9.10 Sequence number
Taught in 7. What a NAS message looks like on the wire (CT1 — the group that writes what your phone says, in depth), 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
This IE includes the NAS message sequence number (SN) which consists of the eight least significant bits of the NAS COUNT for a SECURITY PROTECTED 5GS NAS MESSAGE message.
The NAS message sequence number (SN) with the eight least significant bits of the NAS COUNT is also included in the Intra N1 mode NAS transparent container IE and in the N1 mode to S1 mode NAS transparent container IE.
The usage of SN is specified in clause 4.4.3.