The FPU can be driven on its own. When describing a truck it is remarked by the combination of both units, for example, an MK48 FPU attached to an MK18 RBU is called a "48/18".
The FPU can be operated independently. For trucks, this combination is noted; for example, an MK48 FPU paired with an MK18 RBU is referred to as a "48/18".