It’s like any other WDW room discount. It is not a blanket discount that includes all dates, room types & resorts. It is meant to increase booking for unsold rooms. The included dates get announced periodically, AP discounts are not necessarily announced at the same time as other discounts. Resorts & room types will vary by date & change as inventory changes. Just because a resort is included doesn’t mean discounted rooms will be available there for every night of the promotion. Discounted rooms can & do get sold out. Already having a room booked doesn’t have any effect on being able to apply the discount. Like other discounts tho, the % off varies by resort category. The current spring discount was 25% for values, 30% for moderates & 35% for deluxe & deluxe villas.