They're not given for AS levels, only for full A-levels. Therefore a uni wouldn't know whether someone had got one or not until results day, when they'd already given them an offer or rejected them, unless they applied post A-level. But Cambridge ask for all your UMS marks when you apply anyway. I think how they work is that everyone who gets 600, 599, 598, 597 or 596 UMS marks out of 600 gets one, but it seems to be only AQA that do them for some reason. If you get a really high UMS mark at AS and apply to Cambridge, or get a really high UMS mark at A-level and apply afterwards, they might give you an advantage, but that's pretty much it.