3GPP 23.501 v20.2.0 — the document's own text
5.8.2.19.1 General
Taught in 13. Where the packets actually go (The 5G system architecture, in depth), 5. The connection, and what is promised on it (The 5G system architecture, overview).
5GC supports buffering of UE's downlink packets for deactivated PDU Sessions.
Support for buffering in the UPF is mandatory and optional in the SMF.
When the UP connection of a PDU Session is deactivated, buffering in UPF can be activated by the SMF. If the SMF supports buffering capability, the SMF can decide to activate buffering in SMF instead of buffering in UPF.