A “Detoation?”

As far as I can tell, they're for tagging to make it easier to clarify which enemy you're targeting (this is primarily useful in groups against raid content, obviously). The letter keeps looping a-z while the group structure you're in remains unchanged; joining a group or adding a new member to the group resets the count.

(Not 100% sure about the last one; will test it out when I get home, assuming the server lets me in...)

The rough formula does something like this for a DRG (other classes might have slightly different variances, depending on the attack used):

Damage = (Potency/100) x [(0.0032 x Attribute + 0.4162) x (Weapon Damage) + (0.1 x Attribute – 0.3529)]

For more detailed information, take a look at this Google Doc for more detailed testing, math, and statistics behind the game's battle system.