Basically how quickly the flights are selling. Those airports with a high rate of sale (basically percentage of seat capacity sold over a given period of time) may be in line for more capacity than those with a low rate of sale who may lose capacity. But you also have to factor in area of country as Northern airports tend to sell more early on than Southern ones but this effect works itself out eventually as Southern one’s catch up. Also long haul sells earlier than short/medium haul. Each tour operator will have its own expectation at to where it expects to be at a particular point in time for a particular airport and will also need to account for new route effect, route mix etc. when analysing and comparing. An interesting science for all concerned.