Editing My 3GPP 24.501 notes (section)

=== 4.8.4 Core Network selection for UEs not using Cellular Internet of Things (CIoT) 5GS optimizations ===
If the UE is capable of both N1 mode and S1 mode, when the UE needs to use one or more functionalities not supported in 5GS but supported in EPS and the UE is in 5GMM-IDLE mode, the UE may disable the N1 mode capability for 3GPP access (see subclause 4.9.2).

If the UE is capable of both N1 mode and S1 mode and lower layers provide an indication that the current E-UTRA cell is connected to both EPC and 5GCN without also providing an indication that a target core network type was received from the NG-RAN, the UE shall select a core network type (EPC or 5GCN) based on the PLMN selection procedures as specified in 3GPP TS 23.122 and provide the selected core network type information to the lower layer during the initial registration procedure.

If the UE is capable of both N1 mode and S1 mode and the lower layers have provided an indication that the current E-UTRA cell is connected to both EPC and 5GCN and an indication of whether the network supports IMS emergency services via either EPC or 5GCN or both (see 3GPP TS 36.331 [25A]), the UE selects a core network type (EPC or 5GCN) as specified in 3GPP TS 23.167 [6] annex H.2 for initiating emergency calls when in the state 5GMM-DEREGISTERED.LIMITED-SERVICE or EMM-DEREGISTERED.LIMITED-SERVICE.
:NOTE 1: If the PLMN selection information provisioned in the USIM does not contain any prioritization between E-UTRAN and NG-RAN for a PLMN, which core network type to select for that PLMN is up to UE implementation.

If the UE is capable of both N1 mode and S1 mode and lower layers provide an indication that the current E-UTRA cell is connected to both EPC and 5GCN with:
# an indication that target core network type EPC was received from the NG-RAN, the UE shall select the EPC and proceed with the appropriate EMM procedure as specified in 3GPP TS 24.301; or
# an indication that target core network type 5GCN was received from the NG-RAN, the UE shall select the 5GCN and proceed with the appropriate 5GMM procedure.
:NOTE 2: The NG-RAN can provide a target core network type to the UE during RRC connection release with redirection (see 3GPP TS 36.331 and 3GPP TS 38.331).