3GPP 24.501 v20.0.0 — the document's own text
8.3.14.5 5GSM congestion re-attempt indicator
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
The network may include this IE only if it includes the Back-off timer value IE and the 5GSM cause value is either #67 "insufficient resources for specific slice and DNN" or #69 "insufficient resources for specific slice".