3GPP 23.501 v20.2.0 — the document's own text

5.18.4 Network selection by the network

Taught in 26. Relays, satellites, femtos and shared networks (The 5G system architecture, in depth).

The NG-RAN uses the selected PLMN/SNPN (provided by the UE at RRC establishment, or, provided by the AMF/source NG-RAN at N2/Xn handover) to select target cells for future handovers (and radio resources in general) appropriately. The network should not move the UE to another available PLMN/SNPN, e.g. by handover, as long as the selected PLMN/SNPN is available to serve the UE's location.

In the case of handover or network controlled release to a PLMN in a shared network:

When multiple PLMN IDs are broadcasted in a cell selected by NG-RAN, NG-RAN shall select a target PLMN, taking into account the prioritized list of PLMN IDs provided via Mobility Restriction List from AMF.
For Xn based HO procedure, Source NG-RAN indicates the selected PLMN ID to the target NG-RAN, see TS 38.300 [27].
For N2 based HO procedure, the NG-RAN indicates a selected PLMN ID to the AMF as part of the TAI sent in the HO required message. Source AMF uses the TAI information supplied by the source NG-RAN to select the target AMF/MME. The source AMF should forward the selected PLMN ID to the target AMF/MME. The target AMF/MME indicates the selected PLMN ID to the target NG-RAN/eNB so that the target NG-RAN/eNB can select target cells for future handover appropriately.
For RRC connection release with redirection to E-UTRAN procedure, NG-RAN decides the target network by using PLMN information as defined in the first bullet.

A change in serving PLMN is indicated to the UE as part of the UE registration with the selected network via 5G-GUTI in 5GS.

In the case of handover or network controlled release to an SNPN in a shared network, the following applies:

When multiple SNPN IDs are broadcasted in a cell selected by NG-RAN, NG-RAN shall select a target SNPN, taking into account the prioritized list of SNPN IDs provided via Mobility Restriction List from AMF.
For Xn based HO procedure, Source NG-RAN indicates the selected SNPN ID to the target NG-RAN, see TS 38.300 [27].
For N2 based HO procedure, the NG-RAN indicates a selected SNPN ID to the AMF together with the TAI sent in the HO required message. Source AMF uses the selected SNPN ID together with the TAI information supplied by the source NG-RAN to select the target AMF. The source AMF should forward the selected SNPN ID to the target AMF. The target AMF indicates the selected SNPN ID to the target NG-RAN so that the target NG-RAN can select target cells for future handover appropriately.

A change in serving SNPN is indicated to the UE as part of the UE registration with the selected network.