Heating Valves -- What do they Control?

[Gil_J]

I recently discovered two heating system valves that were leaking. They were screw type vales, like the 2 remaining, and I replaced them with ball valves. I have 4 valves that are visible and am not sure what each controls. They are pointed to with blue arrows. 2 are pointed to but shown in the next photo. The hoses are a bit difficult to trace. I know that the lines go to the Webasco diesel heater and the engine and who knows were else. There is also a round black device (red arrow) in line with a T fitting and 2 other fittings. What is it and what does it do?