3GPP 23.501 v20.2.0 — the document's own text
5.4.1.2 UE reachability allowing mobile terminated data while the UE is CM-IDLE
Taught in 9. Registration, reachability and paging (The 5G system architecture, in depth), 10. Radio, Wi-Fi, cable — every way in (The 5G system architecture, in depth), 4. Knowing who you are and where you are (The 5G system architecture, overview).
The AMF considers a UE in RM-REGISTERED state to be reachable by CN paging if the UE CM state in the AMF is CM-IDLE state unless the UE applies MICO mode or the UE has indicated Unavailability Period Duration.