It's not quite as simple as that. Essentially, all of a child's scores are combined and then they are grouped in "prior attainment groups" with those of similar scores. Using previous progression data and other bits of information, targets are created based on what a typical child in that prior attainment group in the past has then gone on to achieve.
Generally, however, and this is very rough, I would say:
Mix of scores below 90 - ungraded, 1s or 2s.
Mix of scores around 95 - 2s and 3s.
Mix of scores just below 100 - 3s and 4s
Mix of 100s to 105s - 4s and 5s.
Mix of 106 to 109 - 5s, 6s, maybe 7s.
Mix of 110+ - 7s, 8s, 9s.
Obviously, children achieve both under and over these, depending on many factors: personal effort, quality of instruction, support, resilience, etc.