What do the letters below each class mean?

Question

On the class selection screen there are four small colored letters shown below each class. What do these letters mean?

Answer 1

The letters are rankings for the class's offense, defense, healing, and difficulty of player control respectively. You can view them in more detail with ZR (Display Class Info). This screen also says the current class rank and how quickly the character will rank up with this class in the top right.

This is explained in the tip "Fostering Classes"