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

5.34.7.3 AMF implicit subscription about events related with the PDU Session

Taught in 26. Relays, satellites, femtos and shared networks (The 5G system architecture, in depth).

When creating an association with a SMF or I-SMF for a PDU Session, the AMF implicitly subscribes to SMF / I-SMF about events related with the PDU Session (the AMF provides the relevant notification information to the SMF or the I-SMF respectively). This implicit subscription is implicitly released when the corresponding association with the SMF / I-SMF is removed (e.g. as no more needed due to a I-SMF insertion / change / removal).