What must always be included in an abbreviated clearance?

An abbreviated clearance must always include the assigned altitude because this is crucial for ensuring the safe vertical separation of aircraft under air traffic control. The assigned altitude provides pilots with the specific height at which they are expected to operate following takeoff and throughout their initial flight phase, which is vital for maintaining an orderly flow of traffic and avoiding potential collisions. This information helps pilots understand their immediate altitude requirements, which can be critical during busy operational periods.

While flight numbers, departure airports, and destination airports can provide useful context and information for controllers and pilots, the assigned altitude is the most critical element in an abbreviated clearance to ensure the aircraft operates safely within controlled airspace.