What does this valve do?

[lisabass1992]

We have a new to us 1996 airstream classic and when I checked my manual about the black tank flush it mentions this valve under the bathroom sink but doesn't really explain what I should do with it. Thanks in advance for your help.

[A W Warn]

It looks like a vacuum breaker.
If it is, there is nothing for you to do.
Its a valve that lets air into a place that might hold water because a vacuum is created (usually the tank flush)

If you can look at the printing on the cap it will tell you what it is.

[lisabass1992]

Thank you. I really appreciate your help.