What are the letters near the monsters name?

What are the letters near the monsters name on FFXIV such in this picture? Do they have an specific keybind or they`re merely aesthetic for tagging?

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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...)