What is the object (air valve?) mounted above hot water tank

My TT came with a check valve installed on the downstream side of the anti-syphon valve pictured by BB_TX . The check valve is a separate plumbing fixture screwed right into the outlet side of that anti-syphon valve. Each serves a very different purpose.

If my anti-syphon valve ever causing me any problems (leaking), I will eliminate and just keep the check valve (back flow preventer) in the black tank flush line.