5GS Connection Management states

Two CM states are used to reflect the NAS signaling Connection of the UE with the AMF:

CM-IDLE
CM-CONNECTED

CM-IDLE state

A UE in CM-IDLE state has no NAS signalling connection established with the AMF over N1.
The UE performs cell selection/cell reselection according to TS 38.304 [50] and PLMN selection according to TS 23.122
There are no AN signalling connection, N2 connection and N3 connections for the UE in the CM-IDLE state.
When the UE state in the AMF is RM-REGISTERED, UE information required for initiating communication with the UE shall be stored. The AMF shall be able to retrieve stored information required for initiating communication with the UE using the 5G-GUTI.

When the UE states in the AMF are CM-IDLE and RM-REGISTERED, the AMF shall:

Perform a network triggered Service Request procedure when it has signaling or mobile-terminated data to be sent to this UE, by sending a Paging Request to this UE (see TS 23.502 [3] clause 4.2.3.3), if a UE is not prevented from responding e.g. due to MICO mode or Mobility Restrictions.
The AMF shall enter CM-CONNECTED state for the UE whenever an N2 connection is established for this UE between the AN and the AMF. The reception of initial N2 message (e.g., N2 INITIAL UE MESSAGE) initiates the transition of AMF from CM-IDLE to CM-CONNECTED state.
The UE and the AMF may optimize the power efficiency and signaling efficiency of the UE when in CM-IDLE state e.g. by activating MICO mode (see clause 5.4.1.3).

CM-CONNECTED state

A UE in CM-CONNECTED state has a NAS signalling connection with the AMF over N1. A NAS signalling connection uses an RRC Connection between the UE and the NG-RAN and an NGAP UE association between the AN and the AMF for 3GPP access. A UE can be in CM-CONNECTED state with an NGAP UE association that is not bound to any TNLA between the AN and the AMF. See clause 5.21.1.2 for details on the state of NGAP UE association for an UE in CM-CONNECTED state. Upon completion of a NAS signalling procedure, the AMF may decide to release the NAS signalling connection with the UE.

In the CM-CONNECTED state, the UE shall:

Enter CM-IDLE state whenever the AN signaling connection is released (entering RRC Idle state over 3GPP access or when the release of the UE-N3IWF connectivity over non-3GPP access is detected by the UE), see TS 38.331 [28] for 3GPP access.

When the UE CM state in the AMF is CM-CONNECTED, the AMF shall:

Enter CM-IDLE state for the UE whenever the logical NGAP signaling connection and the N3 user plane connection for this UE are released upon completion of the AN Release procedure as specified in TS 23.502 [3]
The AMF may keep a UE CM state in the AMF in CM-CONNECTED state until the UE de-registers from the core network.
A UE in CM-CONNECTED state can be in RRC Inactive state, see TS 38.300 [27].

When the UE is in RRC Inactive state the following applies:

UE reachability is managed by the RAN, with assistance information from core network;
UE paging is managed by the RAN.
UE monitors for paging with UE’s CN (5G S-TMSI) and RAN identifier.