User blog comment:B214/Question and Answers 2/@comment-98.26.119.147-20140125015213/@comment-24438220-20140125021710

Current ability is a huge factor, and it appears to have a bit to do with potential, but doesn't seem to factor in balance break or abnormal growth. Issei, for instance, is considered super weak at the beginning, and only would have required 1 piece if not for his Longinus. Kiba only took one knight- a value of 3- but is already at the level of a high class devil, whereas most knights don't have that kind of potential.

Reincarnation gives the standard devil powers like flight and understanding every language, but how much demonic power they get varies. Asia had twilight healing, but was otherwise normal. Yet once reincarnated, she had an aptitude for demonic powers. Magic is different. Even normal humans can use it if they have an aptitude for it, like LeFey. It was developed based on the equations used in demonic powers, and some devils, like Rossweiess, use magic rather than demonic powers.