3GPP 24.501 v20.0.0 — the document's own text

9.6 Procedure transaction identity

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).

Bits 1 to 8 of the third octet of every 5GSM message contain the procedure transaction identity. Bits 1 to 8 of the first octet of every UE policy delivery message contain the procedure transaction identity. The procedure transaction identity and its use are defined in 3GPP TS 24.007 [11].