I am trying to understand the (sometimes subtle) differences between various terminologies concerning flight schedules. What I find difficult to understand is how exactly the scheduled departure time is defined. My initial thought was that it refers to the time where the aircraft leaves the gate, after the boarding of passengers. However, I noticed that this definition is closer to the term "Target Off - Block Time".
Target Off - Block Time:
The time that an aircraft Operator or Ground handler estimates that an aircraft will be ready to startup/pushback immediately upon reception of clearance from the tower.
Could someone clarify this issue?