The aviation industry knows 2 different types of slot which are not related to each other:

• The airport slot is mandatory at coordinated airports for each movement (arrival & departure) and is valid for a specific time on a specific day of the week and for the specific period requested. It is a permission given by the coordinator for a planned operation to use the full range of airport infrastructure necessary to arrive or depart at a coordinated airport on a specific date and time.
  Allocated slot times are based on the planned on-block (arrival) and off-block (departure) times. Actual times of arrival and departure may vary due to operational factors.
• The Air Traffic Management slot (ATC slot) is needed by each ATC regulated departing flight on the actual day of operation to avoid congested airways. This slot is only valid for this specific flight and for a specific departure time window of 15 minutes. In Europe the ATC slots are centrally coordinated and allocated by EUROCONTROL in Brussels.

"Series of Slots"

A "series of slots"’ shall mean at least five airport slots having been allocated for the same or approximately same time on the same day of the week distributed regularly in the same season. Only "series of slots" are eligible for later historic precedence;  requests or changes other than "series of slots" are considered "adhoc slots" and are not eligible for historic precedence.