From Dissidia Wiki

Block is a universal ability in Dissidia 012 [duodecim] Final Fantasy. Both Ground Block and Midair Block cost 5 AP each to be equipped.

Blocking is used to directly defend against incoming attacks. As it has mid priority, it's most common use is staggering opponents who use low priority bravery moves which compose of the majority of the game's moves. Blocking a Ranged Low priority move will cause that move (typically a projectile) to vanish.

The attack priority tables show how Block can interact with other priorities in the game.

Block's active duration is from frames 1 to 38 and the state itself ends after 52 frames. First 5 frames can be cancelled into a dodge (also known as Blodge) or dash (Block-Dash) which is used to perform Dash feints.

If the player gets staggered from after being blocked or clashing with a melee move of the same priority, the stagger will last for 100 frames. This state can be cancelled with a dodge after 85 frames.

During stagger, if the player has full EX meter and presses R + Square they will activate EX Mode instead of EX Revenge despite stagger being a state during which the player cannot act (without meter) and is susceptible to damage. Blocking a move and then calling the Assist can be used to bypass final EX Revenge situations as assist characters can deplete EX meter on hit.

Precision Block

AP Cost: 45
Blocks are more effective when well-timed.

If the player successfully guards a moves with a block within the first four frames of the block, the extra ability Precision Block will trigger. This will technically turn a regular block into a high priority block, as it will stagger against any mid or high priority move alike. Both the player who used Precision Block and the opponent who got blocked with it will become staggered.

Precision Block will still ignore special or unblockable priority moves, however. With it's expensive 45 AP cost it can be difficult to justify using it in many cases. However, it can be used to nullify Aerith's Holy.