Editing My 3GPP 24.501 notes (section)

=== 5.3.1 5GMM modes and N1 NAS signalling connection ===

==== 5.3.1.1 Establishment of the N1 NAS signalling connection ====
When the UE is in 5GMM-IDLE mode over 3GPP access and needs to transmit an initial Non-Access Stratum (NAS) message, the User Equipment (UE) shall request the lower layer to establish an Radio Resource Controller (RRC) connection. Upon indication from the lower layers that the RRC connection has been established, the UE shall consider that the N1 NAS signalling connection over 3GPP access is established and enter 5GMM-CONNECTED mode over 3GPP access.

When the UE is in 5GMM-IDLE mode over non-3GPP access, and the UE receives an indication from the lower layers of non-3GPP access, that the access stratum connection is established between the UE and the network, the UE shall send an initial NAS message, consider the N1 NAS signalling connection is established and enter 5GMM-CONNECTED mode over non-3GPP access.

Initial NAS messages are:
<ol type="a">
<li>REGISTRATION REQUEST message;</li>
<li>DEREGISTRATION REQUEST message;</li>
<li>SERVICE REQUEST message; and</li>
<li>CONTROL PLANE SERVICE REQUEST.</li>
</ol>

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, for the routing of the REGISTRATION REQUEST message during the initial registration procedure to the appropriate core network (EPC or 5GCN), the UE NAS provides the lower layers with the selected core network type information.

For the routing of the initial NAS message to the appropriate AMF, if the UE holds a 5G-GUTI or 4G-GUTI, the UE NAS provides the lower layers with the UE identity according to the following rules...see spec