3GPP 23.501 v20.2.0 — the document's own text
5.8.2.4.1 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).
This clause describes the detection process at the UPF that identifies the packets belonging to a session, or a service data flow.
The SMF is responsible for instructing the UP function about how to detect user data traffic belonging to a Packet Detection Rule (PDR). The other parameters provided within a PDR describe how the UP function shall treat a packet that matches the detection information.