my water pump on 300e is leaking water from a small orifice in its lower part. whats the function of this orifice. and its suppose to be open without any seal.thanks

The impeller shaft seal is leaking. The little hole is an escape route for leakage. It's always open, it's a drain..

You found its purpose...to let you know when the water pump is crapping out. Once you see leakage, it's time to replace the pump.

Another point on worn out water pumps - it seems like the downhill curve on those things isn't predictable. You can have, say, a small leak, and in a week it's a little worse. You project out and say hey this can wait another weekend. But by that next Thursday it's gushing a radiator full of water every half hour.
Chances are you're better off taking care of the repair sooner rather than later.
Harvey