How are my water heaters connected? This time with PLANS!

So this is my 3rd thread on the subject. I wanted to create a new one because the last two focused on how to get cold water out of my hot water tanks. In this I just want to understand the connections.

I managed to get my hands on the plumbing plans for the house but unfortunately I don't really understand what I'm seeing.

The previous threads were merged here http://ift.tt/1QGHw1H

But so you don't have to go through that thread here are the pictures of the water heaters from the previous thread





Possibly a plug possibly a vacuum breaker



Here is the close up of the water heaters section of the plans



Here is the basement section of the plans:



Here is a vertical cross section:



The questions left over from the other thread were:

What do all these valves do?
Are the heaters connected in series or parallel?
As far as I can tell there are no pressure relief valves I speculated that this could be because they're vented to the roof. Could that be the case?
Is it a plug or is it a vacuum breaker?
It was suggested that there is a thermosiphon water recirculating loop. Is that the case?
Currently I have all valves open, is that how I should leave it?

Thanks in advance and sorry for so many questions about plumbing.

How are my water heaters connected? This time with PLANS!