Rarity (レア度 Rea-do) is a scale used in the the Yu-Gi-Oh! Duel Monsters game guides, Perfect Master BOOK Volume 1 and Volume 2, to indicate roughly how likely a card is to drop from opponents after a Duel. Cards are rated with a grade from S to D, with S usually indicating cards that do not drop, A indicating the rarest drops, and so on. The below table shows the general drop rates corresponding with each grade, though exceptions abound, especially at the boundaries between grades.
Of the fifteen secret cards in Duel Monsters, only #354 "Stuffed Animal" is listed; the other fourteen secret cards are assumed to also have an S grade because it is most consistent.
||1/2048 – 10/2048
||11/2048 – 50/2048
||51/2048 – 100/2048