A Uniform Mark Scale, or UMS, is a way of standardising the marking of papers across different examination boards, allowing someone to compare two marks marked by two different examination boards. Grades are then calculated using grade boundaries set at particular UMS scores.
Until 2008, all A Levels in the United Kingdom were based on a 600 UMS points system, including 300 Points attained from the Advanced Subsidiary (AS) Level.
A Level module exams and courseworks are worth a set number of UMS marks. These modules and courseworks for a specific A Level combine to give a UMS mark out of 600. For example, an A level Chemistry course might consist of 4 exams worth 90 UMS and 2 exams worth 120 UMS.
More recently, a new set of syllabuses have been created for teaching from 2008 onward. Many of these subjects will move from 600 to 400 UMS for the A-level, and from 300 to 200 UMS for the AS. There do remain, however, certain A-levels (e.g. Mathematics) that will retain the 600 UMS mark system.
Raw marks awarded in an exam are converted to UMS marks according to the difficulty of the exam paper and the performance of candidates. For example, one year a candidate may only need 62 raw marks to get an A grade (80%), but another year 62 marks may only be equivalent to a B grade (70%). This means that it is possible for candidates to achieve full UMS marks in an exam, even if they didn't receive full marks in the actual exam.