Question: Under what conditions can a User Equipment (UE) initiate an attach or combined attach procedure?
Option 1: When the UE is in idle mode and receives a paging message.
Option 2: As soon as access is allowed in the selected cell for one of the access classes of the UE, or for emergency bearer services, or for access to RLOS.
Option 3: Only when the UE is registered on the network and has an active PDN connection.
Option 4: When the UE performs a Tracking Area Update procedure.
Explanation: According to the NAS protocol, the UE can initiate an attach or combined attach procedure as soon as access is allowed in the selected cell for one of the access classes of the UE. Additionally, the UE may also initiate attach for emergency bearer services or for access to RLOS (Railway LTE Onboard System). This highlights the flexibility and adaptability of the UE's attachment procedures in various scenarios.