The cruise speed is the speed at which an aircraft travels during the cruise phase.
Overriding the cruise speed
When you schedule a flight, the standard optimum cruise speed of the given aircraft type is used. In some cases it can be useful to override this setting, though, for example to make sure the arrival time falls into a free slot. To do so, select the speed for each day in the flight planning interface. The buttons next to the optimum speed allow you to quickly set the minimum or maximum value.
Scheduling without aircraft assignment
By setting a speed override for a flight day that has no aircraft assignment yet, you are essentially planning a hypothetical flight. This can be of use in the connection analyzer or to help build complex, future schedules in general.