Originally Posted by Uncleted
The fitting is between the holding tank and the pump out deck fitting. No overboard pump out. I figured it might be a non-return.
There are actually a few parts between the point A and point B that you describe. The holding tank would have a threaded fitting screwed into the tank with a hose barb on it. There would be a hose connected to the hose barb of the fitting. The hose would then go to the deck plate.
There wouldn't be a purpose for a check valve on the pump out fitting. It might be an inline connector 'hose barb - hose barb'. sometimes people do that to avoid replacing the entire run.