The online ‘Help’ is no help for “Cavity” – rcliking specifically says “No reference”, so I hope they’ll address it (ie, write an entry for it) at some point.
Meanwhile, what exactly is the “Cavity” check box and it’s associated options “World/Screen/Both” doing?
It highlights the edges in different ways so you get a better view of you model in solid view.
It’s worth noting that use of CAVITY can introduction alarming artifacts on the display of faces, but in actuality these are not affecting the geometry.
BUT, it’s alarming. 
