Hangar deck is an option which lets you choose one of several robot sets at the start of a battle. The option becomes available upon reaching level 15 and opening 4 slots of the first hangar set.
When you purchase an additional hangar deck, the number of slots in it doesn't change: for instance, you have 4 slots opened in your first hangar deck, so when you purchase a second one, it will also have 4 slots.
It works like this: at the start of a battle you can choose one of the sets which suits the current battle mode and map best. After choosing the set, it will be impossible to employ other hangar decks in the battle.
You can access hangar decks menu by pressing the center button in the lower part of the screen. Players can have up to five hangar decks. One robot can be used only in one hangar deck.