Epsilon Mutants are unfortunate mutants. Epsilon mutants pretty much have no chance of having a regular life in society due to their major flaws. If that isn't bad enough Epsilon mutants also only have minor superhuman powers. About 20% of mutants are Epsilon Mutants.
Classify mutants as Episilon Level if they have an inhuman appearance or their mutation makes it impossible for them to function normally. Any accompanying "superpowers" would be next to useless. Epsilon mutants are differentiated from Gamma mutants in that a Gamma mutant's inhuman appearance would provide a compensating benefit like superior strength.
Delta Mutants are like Alpha mutants in that they don't have any significant flaws. The only problem is that Delta mutants don't have powers that match an Alpha mutant, or even a Beta or Gamma mutant. At least 50% of mutants are Delta mutants making them the most common type of mutant.
Classify mutants as Delta-Level if they have a normal human appearance, but their mutagenic powers are weaker or only narrowly applicable, though still controllable.
Gamma mutants have very powerful mutations, but they have flaws. Unlike the Beta mutants a Gamma mutant's flaw is a major flaw that makes his or her life very hard. Also, while Alpha and Beta mutants can pass as regular looking humans, many Gamma mutants cannot because they have physical deformities. About 15% of the mutant population is classified as Gamma.
Classify mutants as Gamma Level if their mutation is powerful, uncontrollable, sometimes useful but usually detrimental to living a normal life, such as a monstrously inhuman appearance that can't be switched off at will.