It's an organization tag. They used to show up in multiplayer, but not anymore. You can just type the tag for any organization you're in before your clan tag when you login in-game.

^Exactly

setup>log-in
And instead of your name, write your org, clan and then name.
Like for me, it's;
ORMO[Fa]chamara

oh, nice. I was having the same doubt lately.

Why not anymore? why did they removed it?

because they dont want such long names anymore (atleast i think so)
u were once able to set your clantag etc. yourself with a character limit, if u go over that u even get banned for it (oh well the good old Ldr[GodLike]Blubkill times... got 24h banned for it )

also its not really organisation tag, its just a tag u can add when u want. but will only shown ingame not on the serverlist etc.

but i saw a person with this yesterday O.o

If it was in multiplayer, you didn't. You can't do that anymore.

It is, however, possible in singleplayer/replays.

i swear i saw zoro with TA in front of his name.