Jets fly to a Cost Index (a speed setting in the FMS). The Cost Index is a number (normally 0 to 100, sometimes 999
) that compromises between speed (eg saving maintenance costs, crew costs if paid by the hour) and fuel burn (the slower, the better fuel burn).
A company may publish a specific Cost Index for a sector or may just have a generic "one size fits all" Cost Index.
The crew then inserts the planned CI into the FMS and lets the FMS work out what actual speed to fly. The FMS will then fly the aircraft at that speed.
The planned speed can be overidden by the crew to make up time or to lose time.