You need over 90% UMS in A2 modules, except for maths, where you only need that in Core 3 and Core 4, and your overall A Level needs to be 80%+ UMS
It means you can get A*s with Bs at AS if you work really hard, but it also means if you did really well at AS and mess up at A2 you may not get an A* even if someone has the same UMS as you who did "poorly" in AS and well in A2
It has it's perks and disadvantages