3GPP 24.501 v20.0.0 — the document's own text
8.3.9.21 ECN marking for L4S indication
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 when the network determines the ECN marking for L4S for the 5G-RG.