It is a drain plug. Its there so you dont dump "as much" atf when you drop the pan to change the filter. You should do the filter too of course no different than changing engine oil.

You may have a torque converter plug too, probably not as I think they were phased out.

Only MB fluid for the 722.6 or 722.9 trans is approved. No other Dex III has the additives required.

If the trans has never leaked you can do the repalce what you take out bit. Fill level is critical. MB uses SDS to monitor trans temp and confirm fluid is at 80C internal temp when they check it with the factory dip stick tool.

__________________
Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)