Can someone explain what UMS is?!

I know it's something to do with marks, but it isn't actually marks(??)
It's either just not a GCSE thing or I've lived under a rock (likely the latter). But I feel as if I ought to know what it is because most do, and yet I'm literally clueless. It's probably something really simple and I haven't picked up on it but hey ho
(Going into yr 12 in 2 days and worried this will be another thing I will be lost about)
Help ur girl out?

When A levels are modular UMS is a way of adjusting the marks from different modules so that they can be added together to work out an overall grade.

Most A levels aren’t modular anymore so UMS isn’t used.