The Document Thingy

What is that document Icon looking thingy in the upper left hand corner of the thread posts? When I mouse over it, it says "Default". (????) Some posts have only that Icon in them.

wild guess, mabe its the type of msg the post will accept.. i have a gaming site and i can change default to say, html and i can put more things in the post, but is not good to have as default cuss your site can get hacked ez'er. just a guess.