Question: In dual-registration mode, when a UE is registered in S1 mode only, what are the options for registration in N1 mode?
Option 1: The UE can register with any PLMN.
Option 2: The UE must use the same PLMN to which it is registered in S1 mode or an equivalent PLMN.
Option 3: The UE can register with a different PLMN only if it is not equivalent to the one used in S1 mode.
Option 4: The UE cannot register in N1 mode if it is already registered in S1 mode.
Explanation: According to the text, when a UE is registered in S1 mode only, for registration in N1 mode, the UE shall use either the same PLMN to which it is registered in S1 mode or an equivalent PLMN. This ensures that the UE maintains consistent registration information across both modes.