3GPP 24.501 v20.0.0 — the document's own text
8.2.11.5 Back-off timer value
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
The AMF may include this IE to indicate the back-off timer value when the Payload container IE is included and the Payload container type IE is not set to "Multiple payloads".