3GPP 23.501 v20.2.0 — the document's own text
5.8.2.19.3 Buffering at SMF
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).
When the SMF supports buffering capability and the SMF decided to activate buffering in SMF for the PDU Session, the SMF shall inform the UPF to start forwarding the downlink packets towards the SMF.
When the UP connection of the PDU Session is activated, if there are buffered downlink packets available and their buffering duration has not expired, the SMF shall forward those packets to the UPF to relay them to the UE. These packets are then forwarded by the UPF to the (R)AN.