Vented vs. Non-vented Rad Caps

can someone school me on the difference? i can't seem to figure it out. i went to the parts store today for a 16lb cap and there were two identical caps but one was marked "vented" and the other just said 16lb. i'm not talking about the ones with levers either. these caps appeared identical but upon closer inspection, i noticed that the bottom "plug" type piece would hang freely on one while the other was spring loaded up into the seal. it's my understanding that both types of radiator caps facilitate flow in both directions... perhaps one just does it with less vacuum?
for example, Stant part #'s
No. 11230: 16 lb. cap
No. 11231: 16 lb. cap-vented


from "rockauto"
STANT Part # 11230 {Standard Cap (Open or Closed System) - 16 psi (SAE Range 14-18 psi)

STANT Part # 11231 {Standard Vented Cap (Open or Closed System) - 16 psi (SAE Range 14-18 psi)
Thanks!

A vented cap allows a small coolant flow when coolant is below boiling temperature. The vent is completely closed when boiling temperature is reached. As coolant cools, it contracts and creates a vacuum in the cooling system. This causes the valve to open and coolant in the reserve/over flow tank to be drawn into the radiator. If the vacuum vent is stuck shut, radiator hoses will collapse on cool-down.

This is straight out of the FSM, '95 Ed.

Systems that use overflow bottles need vented ("relief") caps. Otherwise your system will boil over at the cap under normal operating temperatures.

I stumbled across "vented" and "non-vented" radiator caps this morning. Both vented and non-vented are intended to be used with open cooling systems that utilize an open coolant system reservoir. The factory radiator cap for the '96-'01 is a "vented" design while the '91-'95 is non-vented. Jeep seems to use both designs although non-vented is more commonly used. Interesting to read about the operational difference:

https://www.jeepstrokers.com/forum/viewtopic.php?t=4467

The physical difference between the radiator cap designs is that the one-way flow-back valve that allows coolant to be sucked out of the coolant reserve tank as the coolant system volume decrease is held shut by a weak spring on non-vented caps while for vented caps, the return valve has no spring to hold it shut, and instead it is pushed shut by a rapid flow of coolant through the valve towards the reservoir when the coolant system volume changes under heavy load.

An example of a non-vented 16 psi cap that fits the '96-'01 is the Stant 10230.

Examples of vented 16 psi caps are the Stant 10231, Gates 31523, Duralast/Murray/Motorad 7616 or T16.

I'm curious what cap people are typically using.