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

8.3.7.13 Ethernet header compression configuration

Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).

This IE is included in the message:

if the UE wishes to re-negotiate Ethernet header compression configuration associated to a PDU session and both the UE and the network support Control plane CIoT 5GS optimization and Ethernet header compression; or
to negotiate Ethernet header compression configuration associated to a PDU session after an inter-system change from S1 mode to N1 mode.