What is a slot?

The right to one movement in one hour at one airport. Every departure and every arrival needs one.

A slot you hold stays yours through a re-time — moving a rotation between hours you already hold costs nothing, and nobody can take them. But an unflown holding is released back to the pool, which is the real 80/20 rule airports apply.

Busy airports at busy hours are expensive. That is the game telling you a 6am departure from a quiet field is cheaper than a 9am one from a congested hub.