cold level in the coolant reservoir?

[kikkegek]

i changed my rad to an aluminum version and took a good look at my total cooling system.

I dont understand why there is a cold level in the coolant reservoir?

isnt the only function of the reservoir "overflow"? so excessive coolant can flow freely at too high pressure in the rad?

or does the coolant also get sucked back in?

here is a old pic of my coolant tank, clearly showing the cool level.