Section: 5.2.16.3 Nnssf_NSSAIAvailability service 5.2.16.3.1 General
Content: Service description: This service enables to update the AMFs and the NSSF on the availability of S-NSSAIs and NSAGs on a per TA basis. This service also enables updates for Network Slice Replacement and Network Slice Instance Replacement to the NF Service Consumer (e.g. AMF or NSSF in the VPLMN) when the NSSF determines that an S-NSSAI has to be replaced with an Alternative S-NSSAI or a Network Slice instance is replaced as described in clause 5.15.19 [2]. NOTE: The NSSF can determine the serving network and Access Type from the TAI, as described in TS 29.571[ 5G System; Common Data Types for Service Based Interfaces; Stage 3 ]  [70]. When this service is used by the NSSF for Network Slice Replacement, i.e. to provide to the NF Service Consumer an Alternative S-NSSAI to the S-NSSAI to be replaced, the following cases are possible: - in non-roaming and in roaming case, NSSF in the Serving PLMN provides to the AMF the Alternative S-NSSAI of the Serving PLMN; or - in roaming case, the NSSF in the HPLMN provides to the NSSF in the VPLMN the Alternative HPLMN S-NSSAI; and the NSSF in the VPLMN provides to the AMF the Alternative HPLMN S-NSSAI. Following NSSF event subscriptions are supported: - NSSAI availability status change; - Network Slice Replacement (see clause 5.15.19 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]); - Network Slice Instance Replacement (see clause 5.15.20 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]).