What does scheduled departure time exactly mean?
&quot;Scheduled departure time&quot; is not a term used in the operational environment. It is a term you will find on the passenger side of things, and means the time at which the aircraft is scheduled to leave the departure gate.
There are several different operational terms related to departure time:

ETD, Estimated time of departure. The estimated time at which an aircraft will become airborne.
SOBT, Scheduled off block time. The time that an aircraft is scheduled to depart from its parking position. More or less identical to &quot;scheduled time of departure&quot;.
EOBT, Estimated off block time. The estimated time at which the aircraft will commence movement associated with departure.
TOBT, Target off block time. The time that an aircraft operator / handling agent estimates that an aircraft will be ready, all doors closed, boarding bridge removed, push back vehicle present, ready to start up / push back immediately upon reception of clearance from the TWR.
AOBT, Actual off block time. The time the aircraft pushes back / vacates the parking position.
TSAT, Target start up approval time. The time provided by ATC taking into account TOBT, CTOT and/or the traffic situation that an aircraft can expect to receive start up / push back approval.
ETOT, Estimated takeoff time. Forecast of time when aircraft will become airborne taking into account the EOBT plus EXOT.
EXOT, Estimated taxi-out time. The estimated time between off-block and take off. This estimate includes any delay buffer time at the holding point or remote de-icing prior to take off.
TTOT, Target takeoff time. The Target Take Off Time taking into account the TOBT/TSAT plus the EXOT. Each TTOT on one runway is separated from other TTOT or TLDT to represent vortex and / or SID separation between aircraft.
CTOT, Calculated takeoff time.A time calculated and issued by the Central Flow Management unit, as a result of tactical slot allocation, at which a flight is expected to become airborne.
ATOT, Actual takeoff time. The time that an aircraft takes off from the runway.
ATD, Actual time of departure. Used either to have a meaning identical to ATOT, or AOBT, depending on context.

I've probably missed some, too.