3GPP 23.501 v20.2.0 — the document's own text
5.15.7.4 Support of Network Slice usage control and Interworking with EPC
Taught in 17. One network, many networks (The 5G system architecture, in depth), 6. One network behaving like several (The 5G system architecture, overview).
As described in clause 5.15.15, if Network Slice usage control is required for a PDN Connection, the SMF+PGW-C configures PDU Session inactivity timer to the UPF+PGW-U. When the SMF+PGW-C receives inactivity report of the PDN Connection from the UPF+PGW-U, the SMF+PGW-C releases the PDN Connection.