So the class system for the mutations work’s that at a young age, people will be tested for the mutant gene, which is an additional chromosome that anyone in the universe can be born with. This will then give an idea on whether they can actually develop any sort of powers from the mutation or if it just part of their DNA, it isn’t however a confirmation if the individual will ever actually develop any powers or not however.
Then it’s just a matter of regular test overtime to see if any powers start to manifest, for anyone born with physical mutations that might indicate powers (e.g. MC swirly eyes) than the first part is missed out and it’s more just the regular test to see what powers might be manifesting. Once the power does manifest it weighted against other individuals with similar powers to get a baseline and potential for growth into a more powerful variant and that’s where the actually letter class comes from. This can then change overtime if the individual is re-tested.
The specifics of the test heavily depend on what the examiner’s believe that the manifesting power is, so it’s a system that is littered with flaws. However it is currently the best way to classify how useful a individual might be in a work setting and was picked up as the standard for medical practitioners.