Soft Groups are similar to regular groups, but there are also some key differences between them.
Soft Groups link together elements on different layers. When you select one of the elements in a Soft Group, the linked elements on the other layers are selected as well.
If you move or resize a soft-grouped button on the "MouseOff" layer, the same changes are automatically applied to the buttons linked to it on the "MouseOver" layer.
Text elements can also be soft-grouped. Doing this will automatically apply all changes to each text element in the group, regardless of which layer they are on.