Section: 4.8.4A.1 Core network selection
Content: A UE that supports CIoT optimizations performs core network selection (i.e. it selects EPC or 5GCN) if the lower layers have provided an indication that the current E-UTRA cell is connected to both EPC and 5GCN as specified in 3GPP TS 23.501[ System architecture for the 5G System (5GS) ]  [8]. When selecting a PLMN as described in 3GPP TS 23.122[ Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode ]  [5], the UE shall select a core network type (EPC or 5GCN) based on: a) indication from the lower layers about the CIoT EPS optimizations supported in EPC; b) indication from the lower layers about the CIoT 5GS optimizations supported in 5GCN; c) the CIoT EPS optimizations supported by the UE; d) the CIoT 5GS optimizations supported by the UE; e) the UE's preferred CIoT network behaviour for EPC; and f) the UE's preferred CIoT network behaviour for 5GCN. The UE shall provide the selected core network type information to the lower layer during the initial registration procedure.