Nobody knows what sub levels mean in maths - they're produced because management expect to see "progress" expressed that way, so something has to be made up to placate them. The National Curriculum defines what each of the levels mean (see http://www.education.gov.uk/schools/teachingandlearning/curriculum/primary/b00199044/mathematics/attainment/ma4 ). Sub levels are often produced from test results... Suppose in a test, a score of more than 40 but less than 70 is a level 3 then you might use letters to indicate how well a child has performed within that band. Some schools use "low", "secure" and "high".