Truck diesel filter is installed some where on fuel line, and mainly used to ensure the fuel is clean. Generally, there are two filters on trucks, thus the coarse filter and fine filter.

The coarse fuel filter is installed before truck low fuel pump, normally the located on engine or chassis, to filter water and the big impurities. Due to the existing fuel quality, water is inevitable on all fuel, and it will rust the fuel injector and affect the engine working. So the fuel water separator is to get water out with the theory of density difference and gravity settling. 
The location of fine filter is between low fuel pump and high fuel pump, the precision is about 3-5μm. Without qualified fine filter, the injector may get blocked by the solid particle impurities, which will result the engine operation failure an huge service charge.

Actually poor quality filter can't filtration at all,to protect your diesel engine,it's better to choose Genuine fleetguard filter,Parker Racor filter etc.