Final Fantasy XIII - Active Time Battle
ATB or Active Time Battle is the bar displayed in the lower left side of the
battle screen above the commands. Each command needs a certain amount of
ATB Slots. Lightning starts out with an ATB Gauge of 2 which means she can
use commands that use 2 or less ATB Slots. As the game proceeds, the characters
will get more ATB Slots. ATB Slots refill after they are used during battle.
You can select commands before the ATB Gauge is filled up but they won't be
executed until the ATB Gauge is full. If you don't want to wait, you can press
the triangle/Y-button button to execute the commands that already have a
filled up ATB Slot.
Example: If the character has 4 Slots available, you can select commands until
the slots are filled up. Which means 4 commands of 1, 2 commands of 2 or 1
command of 3 and one command of 1, and any combination there of, as long as the
total does not exceed the total of 4. If you choose one command that requires
3 slots and you don't want to wait until the 4th is filled, you can press
triangle/Y-button to execute the command.