A UE needs to register with the network to get authorized to receive services, to enable mobility tracking and to enable reachability. The UE initiates the Registration procedure using one of the following Registration types:

• Initial Registration to the 5GS;
• Mobility Registration Update upon changing to a new Tracking Area (TA) outside the UE's Registration Area in both CM-CONNECTED and CM-IDLE state, or when the UE needs to update its capabilities or protocol parameters that are negotiated in Registration procedure with or without changing to a new TA, or a change in the UE's Preferred Network Behaviour that would create an incompatibility with the Supported Network Behaviour provided by the serving AMF, or when the UE intends to retrieve LADN Information, or with NR satellite access upon changing to a suitable cell indicating multiple TAs for the RPLMN all of which are outside the UE's Registration Area in both CM-CONNECTED and CM-IDLE state, or when the Multi-USIM UE needs a new 5G-GUTI assignment, or when the UE needs to indicate or returns from an Unavailability Period (see clause 5.4.1.4 of TS 23.501), or when the UE using a RAN that provides discontinuous coverage (e.g. for satellite access with discontinuous coverage) is about to leave the satellite network coverage as described in clause 5.4.13.1 of TS 23.501, or when the UE has informed the network it is unreachable and now returns to coverage using either satellite or terrestrial access as described in clause 5.4.1.4 of TS 23.501; or
• Periodic Registration Update (due to a predefined time period of inactivity); or
• Emergency Registration; or
• Disaster Roaming Initial Registration, as specified in clause 5.40 of TS 23.501; or
• Disaster Roaming Mobility Registration Update, as specified in clause 5.40 of TS 23.501; or
• SNPN Onboarding Registration allows the UE to access an ON-SNPN for the purpose of provisioning the UE with SO-SNPN credentials to enable SO-SNPN access. SNPN Onboarding Registration is only applicable for registration with ON-SNPN i.e. when the UE uses PLMN credentials for accessing an ONN the UE initiates an Initial Registration. The SNPN Onboarding Registration is specified in clause 4.2.2.2.4.

The General Registration call flow in clause 4.2.2.2.2 applies on all these Registration procedures, but the periodic registration need not include all parameters that are used in other registration cases. The following are the cleartext IEs, as defined in TS 24.501 that can be sent by the UE in the Registration Request message if the UE has no NAS security context:

• Registration type;
• SUCI or 5G-GUTI or PEI;
• Security parameters;
• additional GUTI;
• 4G Tracking Area Update;
• the indication that the UE is moving from EPS;
• PLMN with Disaster Condition;
• if the UE is registering with an SNPN, the NID of the SNPN that assigned the 5G-GUTI.

Aspects related to dual registration in 3GPP and non-3GPP access are described in clause 4.12. The general Registration call flow in clause 4.2.2.2.2 is also used for the case of registration in 3GPP access when the UE is already registered in a non-3GPP access and vice versa. Registration in 3GPP access when the UE is already registered in a non-3GPP access scenario may require an AMF change, as further detailed in clause 4.12.8. The general Registration call flow in clause 4.2.2.2.2 is also used by UEs in limited service state (see TS 23.122) registering for emergency services only (referred to as Emergency Registration), see clause 5.16.4 of TS 23.501. During the initial registration the PEI is obtained from the UE. If the PEI is needed (e.g. for EIR check), the AMF shall retrieve the PEI when it establishes the NAS security context with a Security Mode Command during initial registration. The AMF operator may check the PEI with an EIR. If the PEI was retrieved by the AMF (either from the UE or another AMF), AMF shall provide it to the UDM using Nudm_UECM_Registration in order to ensure that the UDM always has the latest PEI available e.g. for reporting event Change of SUPI-PEI association. The AMF passes the PEI to the UDM, to the SMF and the PCF. The UDM may store this data in UDR by Nudr_SDM_Update. During the registration the Home Network (or Credentials Holder in case of access to an SNPN) can provide Steering of Roaming information to the UE via the AMF (i.e. a list of preferred PLMN/access technology combinations and/or Credentials Holder controlled prioritized lists of preferred SNPNs and GINs and/or Credentials Holder controlled prioritized lists of preferred SNPNs and GINs for accessing Localized Services or HPLMN/Credentials Holder indication that 'no change of the above list(s) stored in the UE is needed'). The Home Network can include an indication for the UE to send an acknowledgement of the reception of this information. Details regarding the handling of Steering of Roaming information including how this information is managed between the AMF and the UE are defined in TS 23.122. The AMF determines Access Type and RAT Type as defined in clause 5.3.2.3 of TS 23.501.