This setting controls how far in advance customers can make bookings. For example, if it's set to 90 days, customers won’t be able to book more than 90 days from today. If a specific cutoff date is set, such as April 1st, bookings won't be allowed beyond that date or past the 90-day limit- whichever comes first. You can adjust this in Settings > {Venue Name} > Customer Booking Options.
The multi line box allows you to place a custom message for visitors when they try to book past the advanced booking period. The standard message will show how many days in advance they can book.