Well, the cards are generally separated into 4 items: Digimon, Character/Tamers/Digidestined, Powerups(Items, Equips, etc. Greymon's shield falls into this category), Evolution (Grade A to E). For the sake of the RP though, I think we can safely discard the character cards since they would be pretty dang useless.
As for the Evolution card Grades, the breakdown are as follow:
A: Can Evolve Any digimon level straight to Mega (The equivalent Warp Evolution, subtituted by Biomerge)
B: Can Evolve Any digimon by 1 stage, If used by rookie, they skip 1 level and go to Ultimate (Example: Matrix Evolution in Tamers)
C: Can Evolve any digimon by 1 level, including Ultimate Level Digimon to Mega
D: Can Evolve any digimon by 1 level, except Ultimate Level Digimon to Mega
E: Can Evolve any digimon below Champion by 1 level
PS: I played Yu-Gi-Oh too XDDD