3GPP 24.501 v20.0.0 — the document's own text
8.3.9.11 ATSSS container
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
The IE is included in the message when the network needs to indicate that the ATSSS parameters of the MA PDU session are modified.