939 Series "Personal Heater Vent Valve" ?

In the -10 TM, on page 0004-18, if you look at KEY #1 it points to and talks about
a "Personal Heater Vent Valve" . It says to turn it clockwise to purge
personal hot water heater of air.

I don't get it...what am I missing here? What does it do and why may I ask?

That would be the "personnel heater vent valve", not "personal heater vent valve". It's just a little drain valve, like a radiator drain valve or the air tank drain valves. It's located on the heater box inside the engine compartment. It's at a high point in the heater core, and lets you bleed out any air from the heater core. You turn it counterclockwise to open it, not clockwise.

My guess is that that both hoses have shut off valves at the engine and to purge the air in the heater system when shut off till coolant flows out to keep from having an air pocket going to the engine when heater is turned on. I have never used that valve or the heater yet but I will go looking for it as I don't recall seeing it?

Some cars have bleeder screws at the t-stat housing/water outlet to purge air and I am thinking the same function?

I haven't looked inside the engine compartment in a few weeks, but based on that drawing in the TM and the pictures I have handy of my truck's engine compartment, I think that valve isn't easily visible from the ground. I think you'll need to climb up into the passenger side of the engine compartment to put eyes on it.

The heater system does have shutoff valves at the engine. I haven't been inside the heater box on my 5-tons yet, but I'm guessing that the heater core inlet and outlet are below the top of the core, so an air pocket can accumulate in there if any air gets into the system. Thus, the vent valve to bleed it, just like the thermostat housing bleed screw that juanprado mentioned seeing on some cars.

Yes there is a purge valve on the heater core and two on and off valves, on the 939 and 809 series trucks