SAP is our method of changing schedules around once lines have been awarded, and it happens in two phases.

In SAP 1, you can choose to drop trips for your line or pick up trips from open time, as long as the trips you add would be legal (in terms of duty/rest times), you don't drop below the min guarantee, and those choices are awarded based on seniority. SAP 2 is simply first come, first serve for dropping and adding trips, so its a bit more effective as a way for junior line holders to make changes to their schedule.