What are holdings and what are they used for?

A number of ‘holdings’ or ‘stacks’ have been designated at some distance from Schiphol. These are intended to keep incoming aircraft in a holding pattern at very busy times and during unforeseen weather changes. These aircraft circle in a holding pattern until they receive clearance from air traffic control to land.
In the holding, the aircraft are ‘stacked’, as it were. They fly in a fixed patter there, on 'race tracks', maintaining a minimum vertical distance from each other of 1,000 feet (more than 300 metres).