turret numbers? what 4?

Every time i use any kind of turret there is a little box with a number in it anywhere from one digit to three.
So what the heck is the number for anyway? Is it some kind of heat monitor? Ammo counter? Or maybe some twisted kill counter that boasts how many poor little Multiwinians that turret has murdered? I have also noticed negative numbers and "FF!" on the backs of the flame turrets. So what is it for? Anybody know?

Kill count.

In fact, some brag about their highest kill counts on those turrets :p.

Since it hasn't been answered yet, "FF" means Friendly Fire. You only see it on Flame Turrets and Rocket Turrets since those are the only ones capable of killing friendly DGs.