If you're talking about that half pocket on the other side of the box call, I use that for my crow call, cheater glasses, and or rubber cover for my FF3.
I got blood on the back of my vest. It dried and I took a hose to it... it sprayed off w no stain. I probably have 20 hunts wearing mine. My water system has turned into a 20 oz plastic flask I bought off amazon. The flask can fit in the left inside pocket or of course, back of the vest.