There is one basic Maths GCSE.
For that GCSE you can take harder exams if you are good at maths. That is called the Higher tier.
Or if not so good at maths you take easier exams called Foundation tier. If you take the Foundation exams you can't get above a new grade 5 (a low B in previous grading).
If you take the higher paper but can't do it instead of getting a low grade you can end up ungraded U.
Either tier you end up with 1 final overall grade and it doesn't say on your certificate which tier of papers you took.
The school should decide/recommend what tier a pupil is entered for. For the top and bottom sets this will be obvious. For some pupils in the middle it may not be decided until the spring of y11.