In Sign Up Genius, slots are used to manage user registration. When a user registers for the first time, they are assigned a slot. After they sign up, they will be placed in one of the available slots. When they create an account, they will be automatically placed in the first available slot.
Slots are also used to manage users who have already registered. If a user has more than one account with Sign Up Genius, they will be placed in the slot that corresponds to the most recent account they created.