NEVER the object will be hidden however it will still be part of the container, so if you are in a stack group and you have an object in the middle you will see a blank area since the space used by it will still be there, so "never" is very much like "invisible"
REMOVED the object will be entirely removed from the layout, so, if it was using any space that space will be freed. In a stack group if you remove an object the stack will be changed and there will be no space left between the items
In both cases all formulas inside the layers will still be updated, only draw will be skipped