Shell on shell off? Newbie

Can y'all please explain the difference btwn shell on repair and off. I have seen a lot of posts about shell off and read a few just not sure about what shell on entails. My project is pretty new all I've done is clean it. It has already been gutted except the bathroom. The only place I can see floor root is in the bathroom. There is also a piece of belly pan? (I think that's the right term) that is unattached at the rear don't have any specific pics of it but will upload some ASAP. Just trying to read up now. Any help greatly appreciated