3GPP 24.501 v20.0.0 — the document's own text
6.2.5.1.2 Session-AMBR
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
The NAS protocol enables the network to provide the UE with the session-AMBR associated with a PDU session.
The standardized value of two seconds is used as the averaging window for the UE's enforcement of the UL rate limitation indicated by the session-AMBR.