3GPP 23.501 v20.2.0 — the document's own text
5.15.11.3.3 HPLMN NSAC Admission Mode
Taught in 17. One network, many networks (The 5G system architecture, in depth), 18. Being served by somebody else's network (The 5G system architecture, in depth), 6. One network behaving like several (The 5G system architecture, overview).
In this admission mode the AMF or SMF in VPLMN interacts with HPLMN for admission, both for number of registered UEs or the number of LBO PDU sessions respectively.
For each S-NSSAI of the HPLMN that is subject to NSAC and mapped to a corresponding S-NSSAI of the VPLMN, AMF performs NSAC admission for the number of registered UEs with the HPLMN central or primary NSACF for all inbound roamers from that HPLMN when they register in this VPLMN. The AMF discovers the HPLMN primary or central NSACF or be configured with the needed information.
For each S-NSSAI of the HPLMN that is subject to NSAC and mapped to a corresponding S-NSSAI of the VPLMN, every SMF in this VPLMN performs NSAC admission for the number of LBO PDU sessions with the HPLMN central or primary NSACF for all inbound roamers from that HPLMN when they initiate an LBO PDU session. The SMFs discover the HPLMN primary or central NSACF or be configured with the needed information. For each S-NSSAI of the HPLMN that is subject to NSAC, the SMF performs NSAC according to the principles described in clause 5.15.11.2 for home routed PDU sessions.
In the HPLMN NSAC admission mode, the primary NSACF or central NSACF in HPLMN determines whether the NSAC admission request for a roaming UE is accepted or rejected.