The derivative parts are, for example, the "symmetrical" parts. Like the piece B is the symmetrical of the A.
So, when you evolve your room A, room B evolves too.
So now, as you have changed versions, I assume that at each opening, Solidworks warns you that the part will evolve around 2015. (No return possible in 2014)
And so he asks you if the "symmetrical" (or other) should also evolve.
If you don't, you lose the connection. If you do it, you keep it
This post is about custom properties, not geometry
Basically, if you have a different custom property "part number" (for example) for room A and B you have to choose "no", if the property is identical "yes". On the other hand, it's valid for all properties...