When issuing a route of flight, which of the following must be specified?

When issuing a route of flight, the specification of a vector is essential because it provides the pilot with specific directional guidance to follow. Vectors are typically used to ensure safe navigation through controlled airspace, to facilitate sequencing and spacing among aircraft, and to manage traffic flows effectively. They allow air traffic controllers to guide aircraft around obstacles, weather patterns, or other traffic in a dynamic airspace environment.

While altitudes, weather conditions, and flight plan numbers are relevant to overall flight operations, they are not always a compulsory part of the immediate route issuance. Altitudes might be provided separately and can be adjusted based on operational needs or airspace constraints. Weather conditions are important for flight safety but do not directly form part of the routing instructions. Similarly, flight plan numbers are administrative details that do not influence the immediate routing of the aircraft. The priority is to give pilots navigational vectors to follow while they operate within the airspace.