3GPP 24.501 v20.0.0 — the document's own text
8.3.14.6 Access type
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 network releases user-plane reources of an MA PDU session specifically over either 3GPP access or non-3GPP access.