What phrase must FSS use to prefix a clearance?

The phrase that must be used to prefix a clearance by Flight Service Stations (FSS) is "ATC Clears." This terminology is essential because it establishes the authoritative source of the clearance, indicating that it comes directly from Air Traffic Control (ATC). This clarity is vital in ensuring that pilots and air traffic personnel understand the legitimacy and origin of the instructions being given.

Using "ATC Clears" connects the clearance to ATC rather than suggesting that FSS is independently issuing the clearance, which might create confusion regarding who has the authority and responsibility for that clearance. It reinforces the chain of command and ensures that pilots comply with the proper protocols, aligning with the established aviation practices and regulatory requirements.