3GPP 23.501 v20.2.0 — the document's own text

5.32.5.3 Access Availability/Unavailability Report

Taught in 11. Using two accesses at once (The 5G system architecture, in depth).

If required by the network in the Measurement Assistance Information, the UE shall provide access availability/unavailability reports to the network. How the UE detects the unavailability and the availability of an access is based on implementation. The CM state of a UE is not a factor when determining whether the 3GPP access is available. When the UE detects the unavailability/availability of an access, it shall:

build a PMF-Access Report containing the access type and an indication of availability/unavailability of this access;
send the PMF-Access Report to the UPF via the user plane.

The UPF shall acknowledge the PMF-Access Report received from the UE.