3GPP 24.501 v20.0.0 — the document's own text
8.3.1.17 Requested MBS container
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 when the UE requests to join one or more multicast MBS sessions that are associated with the PDU session.