What does ‘Efficiency’ mean in the Advanced view of Missions Available?

The ‘Efficiency’ figure helps you determine whether a trip makes sense for you based on how far out of your way you need to fly to complete the mission. For example, if the mission originates or terminates at your home base, the mission would be 100% efficient because you are not going out of your way. The figure decreases as the origin and destination airports move further away from your home base.