An SBI capable IMS entity (e.g. I-CSCF, S-CSCF or IMS AS) performs HSS discovery to discover an HSS that manages the user subscriptions. The SBI capable IMS entity shall utilize the NRF to discover the SBI capable HSS instance(s) unless the information about SBI capable HSS instances is available by other means, e.g. locally configured on the SBI capable IMS entity. The HSS selection function in SBI capable IMS entities selects an SBI capable HSS instance based on the available SBI capable HSS instances (obtained from the NRF or locally configured). An SBI capable IMS entity always selects an HSS within its own PLMN. The HSS selection should consider one of the following factors when available to the SBI capable IMS entity:

1. HSS Group ID of the IMS identity (IMPI or IMPU, or PSI).
2. IMS private identity (IMPI); e.g. the SBI capable IMS entity selects an SBI capable HSS instance based on the IMPI set the UE's IMPI belongs to, configured locally or based on the results of a discovery procedure with NRF using the UE's IMPI as input for HSS discovery.
3. IMS public identity (IMPU); e.g. the SBI capable IMS entity selects an SBI capable HSS instance based on the IMPU set the UE's IMPU belongs to, configured locally or based on the results of a discovery procedure with NRF using the UE's IMPU as input for HSS discovery.
4. Public Service Identity (PSI); e.g. the SBI capable IMS entity selects an SBI capable HSS instance based on the PSI set the received PSI belongs to, configured locally or based on the results of a discovery procedure with NRF using the received PSI as input for HSS discovery.
5. MSISDN; e.g. the SBI capable IMS entity selects an SBI capable HSS instance based on the MSISDN set the UE's MSISDN belongs to, configured locally or based on the results of a discovery procedure with NRF using the MSISDN as input for HSS discovery.

Unless the information about the interface type to be used towards HSS is locally configured on the SBI capable IMS entity, an SBI capable IMS entity can also use the NRF to decide the type of interface (SBI vs diameter) to be used towards HSS. The following clause describes the procedure for HSS registration in NRF, SBI capable HSS discovery and interface type selection via NRF.

An SBI capable HSS registers in the NRF using the Nnrf_NFManagement_NFRegister Request message as defined in TS 23.502. The NF profile of the HSS registered in NRF includes necessary information for an SBI capable IMS entity to send SBI service requests to the selected SBI capable HSS service instance. Different SBI capable HSS instances managing different sets of IMPIs/IMPUs may be deployed in a given PLMN. In this case, the SBI capable HSS instances register in NRF using either different ranges of IMPIs/IMPUs and/or HSS Group IDs.

Annex AB describes deployment options for SNPNs to enable access to IMS services for SNPN subscribers. Two potential deployment options are described below. For the purpose of this Annex, an IMS Subscription Holder (ISH) identifies the administrative domain holding the IMS subscription for a UE subscribed to receive IMS services. The ISH can be the same or independent from the SNPN domain which deploys the 5GS. In the case the ISH is different from the administrative domain of the SNPN, the UE needs to have two separate subscriptions, one for accessing the 5GS of the SNPN (provided by the SNPN or Credentials Holder (CH) as defined in TS 23.501) and one for accessing IMS services provided by the ISH. The UE needs to be successfully authenticated in each domain, using the appropriate subscription, before able to receive IMS services.

In this option, the SNPN deploys its own IMS used by subscribers with a SNPN 5GS subscription. The SNPN and the ISH belong to the same administrative domain. In this deployment scenario, either USIM/ISIM or IMC is used to authenticate and authorize a UE for accessing IMS services.