Indeed, this information is crucial. UniJoin forgot to mention how many slots the raffle will have and this is very important. If people would choose slots from "a" to "f" and the raffle will have 100 slots, those letters won't even fit within the 100 slots, which are made only by numbers.

Perhaps those willing to pick a slot now can pick slots from 0 to 9 (excepting 2, 6 and 7, which were already occupied), as those are for sure part of both types of raffles (be it with 16 slots or with 100 slots).