Yes, whats behind door number #2..
If the shower is indeed on the left, downstream of valve means its a direct drain for shower (this was 30 years ago). If its upstream means valve is inbetween shower and other drains:h The more I looked at picture just didnt compute, IF the dump valve was a dump valve. But it doesnt sound like it. Sounds as if there's another valve, though not a blade valve thats opened to drain. But that only makes sense if there was no holding tank and had hose thread. Possibly shower on the left is upstream of #2. Then #1 makes sense its on its way out except no mention of where its at outside
Otherwise looks like you either open to shower or close to use sink? But just a lot of guessing on my part trying to make whats shown flow...too many missing parameters-need one more better clue mommie53241 :)