The letters A, B, C, D, and E represent how effective the skill is.
In each monster or character, you can change values for the attributes and conditions to A, B, C, D, or E to represent their resilience to that kind of attack or condition.
The damage multiplier, however, is a function that I am unfamiliar with... I haven't used it, nor have I seen it when exploring my RM2k/RM2k3 programs...