Hiding geometry that is connected to other geometry does not 'separate' it - so it moves/deletes as one thing. Using different on/off layers for part of some connected geometry also 'hides' it, BUT again it's still connected. It's a recipe for madness. Make a rectangular face. Select the face and assign it to an 'off' layer. Now erase one of the visible edges. Now switch the 'face-layer' on - the face has vanished as it depended on the edge that you just erased ! Always model raw geometry on Layer0 - have it as your active layer.. To separate parts of geometry use a group [or make a component of it]. You can then hide/layer that 'container' separately from the rest - it is no longer 'connected'... Geometry that sticks together is very useful, but when you don't what that effect put the geometry inside 'container' groups or components...