One is a "heater" fan coil unit that get's it hot water from the engine's coolant that is cycled through it -- like a hot water heater's heat exchanger.

It functions like your car's heater coil with a fan that provides "free" heating when the engine is running.

Secondly, he has a Dickerson heater -- that's a conventional diesel cabin heater that burns diesel fuel. Max want's to protect it from gunk in the fuel messing up its filters. He's got a pretty extreme little filter system in the Racor for that simple Dickerson heater. ;^) [I questioned whether the fuel pump would have a problem overcoming that, but Max thinks not.]