Cambridge International AS & A Level. How are the grade thresholds actually counted?

I am currently taking the exams and i am both confused and worried on how to actually get an A for the papers.

Say that i am taking the Paper 4 for A level which is out of 100 points max, i read in a website that to get an A it has to be 80/100 but that is a UMS mark or something? but in reality the grade threshold for an A is actually 66% like that or something. Can someone please help me on how actually a grade threshold (A,B,C) given? is it fixed like 60/100 is a c and 70/100 is B or something else?