Much like intersect grouping, subtractive grouping requires you to have multiple objects overlapping each other.
When you come to press the subtractive group button
it will remove all the objects from the last object you had picked, i.e. the object highlighted in red. This will result in creating holes in your last object in the shape of the objects you have removed. Again this function only shows up when you rendering the scene.
Before Subtraction
After Subtraction
