Question: When should the Additional GUTI Information Element (IE) be included in a registration procedure?
Option 1: If the UE performs the registration procedure due to inter-system change from N1 mode to S1 mode.
Option 2: If the UE operates in dual-registration mode and has two valid native 5G-GUTIs.
Option 3: If the UE performs the registration procedure due to inter-system change from S1 mode to N1 mode, operates in single-registration mode, and has a valid 5G-GUTI; or if the UE holds two valid native 5G-GUTIs and one of them was assigned by the PLMN with which the UE is performing the registration.
Option 4: If the UE performs the registration procedure due to intra-system change within N1 mode.
Explanation: The Additional GUTI IE should be included in specific scenarios, such as when the UE changes from S1 mode to N1 mode and operates in single-registration mode with a valid 5G-GUTI, or when it holds two valid native 5G-GUTIs and one of them was assigned by the current PLMN. This ensures that the network has the necessary information for registration and authentication purposes.