Hi and welcome,
Well, the "B & B Full way termination Valve" is just a fancy name for your Black Tank dump. You'll find that nobody uses that terminology.
I have to assume that you mean you do not know how to dump the black tank on your Class C motorhome. Is that correct?
I'm also guessing that you found that odd wording on the sticker next to the Black Tank Flush inlet. Correct? If so, all that means is, "Don't hook up a hose to this black tank flush if your black tank dump valve is closed. Otherwise, you can get distracted and the black tank will fill up with water and overflow all over the place and you will HATE IT when that happens.
Did no one show you have to dump the tanks - both black and grey - when you took possession of your motorhome?
Here's a video that shows how to dump and rinse on someone's Fleetwood Class A motorhome. Now, yours will not look like this and probably not have everything located the same way, but look past that and get an understanding of the process;