When you home that sets the machine coords, you can then jog to any point you wish and type a number into the DRO or Zero the DRO via the button and that will be an offset. So if for examply you jog the Z down to your work piece surface and zero the DRO that is now your work offset zero for the Z axis, a move up will be positive, a move down negative.
You can also type directly into a DRO and also with the latest version you can add, subtract, divide and multiply in a DRO. As an example if you wanted to touch off feeler gauge you would jog down so that the axis is just above the work and then slowly jog the axis until the feeler gauge slips in,you can then type the thickness of the feeler gauge into the DRO and that will mean the surface of the work is zero.
Hood