682.
The answer is a bit of both really.
As the Tour op we can request maximum capacity on an aircraft from anywhere. The Airline decide if they will W in from another airport, base an aircraft there or position in for a few days of the week. However, If we only wanted a certain percentage of capacity they could "suggest" an alternative Airport if it was cheaper for them or they could'nt find sharers. Thats assuming that they have a spare aircraft, suitable slots, crew for it, relevant ground support etc.
But in these vertically intergrated days, the decesion should be made by whatever makes the most money for the group.
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You fly em we'll fill em!