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 function on its own. When describing a truck, it's indicated by the use of both units together, like an MK48 FPU combined with an MK18 RBU being termed as a "48/18".