Yes it's suppose to keep any excess fluid from leaking out of the cart when laying on it's side. It does seal pretty well.
I don't use the o'ring or the white cap. Before I pull the white cap off I place one of the silicone covers on there and use an exacto knife to cut a small hole in the center
through the white cap. I then remove the cap and o'ring and toss them in a plastic bag. I
vape through the small hole in the silicone cover. When I need to top off or refill it makes it a lot easier than trying to pull that white cap off.