Dissidia 012 operates with a priority system that decides what move prevails against another move.

A list of priorities

As a general rule, higher priority beats lower, identical Melee priority clashes and Melee beats Ranged if same priority.

Melee attack = attacker will be staggered when blocked
Ranged attack = attacker will not be staggered when blocked
Ranged attack, Melee attack with Low priority -- most bravery attacks
Ranged attack, Melee attack with Mid priority -- cause stagger on normal R block
Ranged attack, Melee attack with High priority -- cause Guard Crush on normal R block, most HP attacks
Special -- Special case of ranged attacks, can not be blocked, but dies to attacks and dashes (e.g. Kuja’s Remote Flare BRV)
Unblockable -- does not collide/touch other attacks/blocks at all. (e.g. Kefka’s Hyperdrive HP)
Dash -- include Quickmoves and any dashes
Block Low priority -- attacks with magic guard such as Squall’s Fated Circle, Cecil’s Dark Cannon, Warrior of Light's and Tidus' EX mode bonus
Block Mid priority -- Normal R block, Warrior of Light’s Shield of Light, Exdeath’s Delta Attack, Firion’s Shield Bash and more.
Block Mid* priority -- Normal R block with Precision Block activated.
Block High priority -- Cloud of Darkness’s [Wrath] Particle Beam, Vaan's EX Mode (SW) Sword & Shield, Feral Chaos' Deus Iratus.
Block Highest priority -- Exdeath Omni Block, Ultimecia’s Charged Knight's Lance, LV2 Assist Change
Jecht Block - Jecht's Jecht Block.

Reference tables

Left-hand columns are the point of comparison.