I don't know about the showcase either, but about the combo, just do exactly what it said, like your example
D, DF, F, a+b = Down then Front then A+B (press both AB button at same time)
Sometime the command is easy to confused like this:
D, F, a = Down, Front, A
D, DF, F, a = Down, DownFront, Front, A
But they basically same. If a character have similar move like that and you press Down, Front, A, they can randomly perform either of them. Normally the creator will avoid create character with similar command like that but some creator just doesn't care at all so it's hard to get the correct move when you press.
