Broadly speaking, we have two parameters we care about when we're deciding what a reaction ability does: the trigger and the effect. There are basically seven different trigger conditions:
- You've been attacked (whether or not the attack hit). This is the most common trigger; 8 of the 27 reactions fall under this umbrella, including Counter.
- You've taken HP damage from an attack (including 0 damage, but not including healing). This trigger also applies to 8 reactions, buth they're somewhat less commonly-encountered reactions, such as Speed Save.
- You've taken HP damage from an attack, and you're at critical health afterward. 4 reactions use this trigger: HP Restore, MP Restore, Critical Quick, and Meatbone Slash (Bonecrusher). Meatbone Slash is a really good ability that's overshadowed by Blade Grasp existing on the same job: it deals damage equal to your maximum HP, making it very likely to kill your attacker.
- You've been targeted by an attack, and the reaction applies before the attack resolves. This is comprised of the 7 reactions discussed in the previous post.
- You've lost HP by almost any means possible, including Poison, fall damage, and even self-inflicted damage from abilities like Wish. This trigger is unique to MP Switch (Mana Shield).
- You've been healed in excess of your maximum HP. This trigger is unique to Distribute (Cup of Life).
- You've been the target of an ability that has a Faith-based effect (including magic and magic guns). This trigger is unique to Face Up (Faith Boost), which is a detail I missed in the original Mega Tact. Granted, this is not an especially competitive ability...
- MP Switch turns HP loss into MP loss. It's hard to call this a standard reaction ability, because it can occur in response to lots of things that aren't actions; on the other hand, like most reaction abilities, it doesn't apply to damage received from other reaction abilities.
- Distribute, that odd duck. I'd love to see someone try to use Distribute effectively. Unfortunately, you can't Distribute healing that you inflict on yourself, but you could get up to some shenanigans with something like Chameleon Robe and Math Skilled Holy.
- Absorb Used MP is Celes's Runic except that the spell still goes off. It's hard to make a case for this, if I'm being honest.
- Counter Magic differs slightly from Counter, Counter Tackle, and Counter Flood in that the ability used is whatever was originally cast. The caster must expend MP to cast the spell.
- Damage Split is the only reaction that cares about how much damage was done.
- HP Restore, MP Restore, and Meatbone Slash seem like they could fall under the umbrella of "reactions that use an ability," but the ability they use doesn't actually exist.