It is overall, not all modules. If you look up your exam boards unuform mark grade boundaries you will grt a pdf with the grade boundaries.
E.g. CCEA grade biundary for scienes (with 3 modules 600 total points) its 520/600 for an A*
Ill use my AS classical civ as an example cause its easy to do - it is 2 modules, both worth 100 marks each. Total for AS and A2 year is 400, an A* is 360/400. I got 182/200 at AS (across the 2 papers - 84% in one 98% in the other). I in A2 had 2 papers again 100 each. So to get what I need this year for an A* It is (360-182) divided by 2. I need 89% in both papersfor an A*. So doing better as AS meant I could afford to drop a bit at A2, since if I didnt get the A* id at least be safe in my A since dinky need 69% for an A. Ive justgiven that as a real life example to prove its overall not in each, if you get 95% in one paper then you can get 85% in the other it would even out to be tge 90 you need. Sorry long winded lol! But remember scaling factors in and its unpredictable... so if you got 80% on a very hard paoer that could be scaled up to 90, its unpredictable what scaling will do...