the tab caps are so that if you do have to take the cap off when the radiator is hot, you can relase the pressure that way, and then remove the cap after it has vented... it's safer than opening a normal cap half way and hoping you don't get hit with boiling water...
it begs the question... why did you buy a lever cap if you didn't know what it was for?
it begs the question... why did you buy a lever cap if you didn't know what it was for?