These screen-cast illustrate how works selection of booking resources with aggregate parameter and different parameters in this shortcode. System set as unavailable the dates in all calendars in all booking resources if for such date(s) has been made the bookings. But if you defined some dates as unavailable in WP Booking Calendar > Availability > Days Availability page or in WP Booking Calendar > Availability > Season Availability page, then system set such dates differently depends on value in option parameter of Booking Calendar shortcode and depends if this "Season Availability" or "Days Availability" settings.
It's give you flexibility in configuration of unavailable dates in booking resources.