Combo time modifier. For example, connecting and comboing with a 5A started would give you a much shorter combo than with 5B. 5A would be faster (and therefore easier) to connect with, but the trade-off is you can't combo for as long. The best approach would be to get an understanding of your moveset first, because then you'll be able to visualize the chain routes and combos much easier. Even though there are a lot of combos, a lot of elements in the combos are repeated and are usually just variations of one another for different situations. For example, you might end a combo with 22C for oki, or you can end it with 214B~C for meter, but the standard combo route midscreen would be ...5B>2B>5C>3C>end. You could add a 2C into that combo (as one of the variations I mentioned) but you have to be close enough and I believe you can't 22C off that because the target would be too far away.
If I'm wrong in any aspect, I hope one of the more veteran players will correct me.