Hi, @Thekatzenjammerkid
@PhotoDad is correct about STEP. It is a much more difficult exam, and I think the other universities using it now mainly do so as an alternative option to the Autumn exams.
STEP is written and marked at Cambridge, who set the grade boundaries nationally. Cambridge are quite open that they do this in such a way as to control their intake: about 50% of Cambridge Maths offer holders fail their offers, mostly because of the STEP component.
To be sure, STEP is by far the hardest UK university admissions exam going. But some part of the failure rate is down to underpreparation. Cambridge Maths applicants are so strong that it can be hard for them to realise what creative and devilish problems can be wrought using concepts from A Level Maths and FM.
But there is a lot of support available, if not from DC’s school then from the Advanced Mathematics Support Programme (or similar, if not in England. End of Y12/beginning of Y13 is the recommended time to start preparing. And of course STEP prep helps with everything else mathematical.
I apologise if you knew this, and hope it may be helpful for someone else.