It's a Taco zone valve schematic. It's showing the maximum allowed zone valves that can be used with one low voltage transformer. 3 zone valves maximum per transformer.
Three of the circles represent zone valves. The other three circles represent thermostats. The other thingy there represents the low voltage transformer.
PS: Taco zone valves have become very reliable during the years. At one time they were considered junk. They simply would not last. Not no more though. They work great!!!