Body modifiers use the three-dimensional (3D) geometry of an object, such as a mass element or a mass group, to add to or subtract from a stair component (tread, riser, or stringer depending on the stair style). Body Modifiers only work on a single instance of a tread, riser, or stringer at a time.
After you create a body modifier from an object, you can delete the original object. However, if you have created a complex object, such as a mass group comprising many mass elements, you may want to retain the object in the drawing until you are sure you have the results you want for the stair.