Coolant overflow hose

2001 Mazda Tribute

Why doesn't the coolant overflow hose connect to anything?
and what do i do with it during a pressure test?
and if the coolant system is truly a pressurized system, why is that hose not connected to anything? Why would the coolant cap have a pressure rating if the overflow hose is always open to air?

I know on my 95 civic i don't remember but i think i have to plug the coolant overflow reservoir during pressure tests. The overflow reservoir on the 95civic is also not completely sealed so i guess it's just i don't have a great understanding of that if someone could explain the relationship between the cap and overflow opening. For example how could the reservoir possibly explode, as i have seen happen, if the overflow hose is always open to air?
 

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