It's all just a part of the boards. It's meant to keep track of the post count. This feature is found on most pHp based boards, although with different terms. If I've counted right, here are the categories:
0-99 Posts: Beginner
100-499 Posts: Drumming Adept
500-999 Posts (I'm guessing on the last number, there): Session Drummer
I don't know what the next category is for over 1000 posts.
Also, you'll notice the green "pips" under your name. That's part of the whole thing, to show what level you are. For example, I recently crossed the 500 post count, so a 3rd green "pip" was added to my status bar, and my title was automatically changed from "Drumming Adept" to "Session Drummer".
It's just a fun little thing, not meant to reflect on anyone's drumming ability. It's meant to let you know who the senior members on the boards are, by post count only.
My opinion on this questions is the following and in no particular order:
The number of years of actual playing experience
The type of gigs that you play (i.e., quality of playing experience not type of music)
The type of players that you play with (i.e., beginners - more experienced)
The number of players/bands that call you back to play with them again
Just being in the right place at the right time (i.e., getting the opportunity)
All of this has to do with your abilty to network with good players, have confidence to step up when an opportunity arrives, and abilty to be responsible and keep a great work opportunity. It is about being lucky sometimes as well aa who you know to get a break.
So I believe there is more to being an "experience drummer" than your ability to play fast, hard, long, use double bass, play with a click track or have an expensive drum kit.
I know this if you keep a positive attitude and keep your ears open to oportunities they will come your way. You just have to not give up if life doesn't deal you the great opportunity early on. Sometimes it takes years to get that break.