On Fox body Mustangs, the system is pressurized on the radiator side of the cap and has only atmospheric pressure on the overflow tank side of the cap. The recovery/overflow/puke tank vents air to make room for the expanded coolant when the coolant pressure exceeds the pressure rating of the cap. (It used to be cars just spit up coolant onto the pavement without any recovery and just sucked air back in.)
As the cooled coolant creates a vacuum in the radiator side of the system, a little spring in the radiator cap allows fluid from the tank to be sucked back in to the pressurized part of the system. Air from the atmosphere replaces the recovered coolant in the overflow tank. If the car really overheats, has the tank overfilled, or has a head gasket problem, it can overflow the overflow tank and still spit up on the pavement and your battery area.