What can be assigned to regulate or restrict departure flows?

Clearance void time is a specific regulation designed to manage departure flows by establishing a time frame within which an aircraft must depart. When a clearance void time is assigned, it indicates that the aircraft's departure clearance is only valid until a specified time. If the aircraft does not depart by this time, it must contact air traffic control (ATC) for a new clearance. This mechanism helps ensure that departures occur in a controlled manner and prevents congestion or conflicts with other flight operations.

In contrast, while flight level restrictions are useful for managing altitude and separation among aircraft in the en-route phase, they do not directly influence the timing of departures. Weather briefings provide information about conditions that may affect flying, but they do not actively regulate flows. Delay notifications inform pilots of expected delays, but they do not impose specific departure time limits. Therefore, the establishment of a clearance void time is the most effective way to regulate and restrict departure flows.