Only one operation that reserve time on the calendar can be processed in the single moment of time in the scope of your account.
This operation always check whether the time is still available prior it will reserve it.
All other reservations are waiting for their turn.
This is achieved by using system locks.
This eliminates the possibility of an accidental overbooking.
The administrator can manually do a overbooking.
To do this, the Overbook checkbox is included in the reservation parameters.
With this checkbox set, you can reserve more shares than is actually available.