This is the setting for how long a package lasts. For some businesses, this is a hard and fast rule. For others, it’s a general average. For example, take a package for a private party that occupies a room for two hours. The duration would be 120 minutes, and the booking interval is 120 minutes. People can book that package at 10 am, noon, 2 pm, etc.
Say you also have non-private packages, and do not put a time limit on them. But, from experimentation, you have learned the average group at your field stays about 2.5 hours, so you would set the duration to 150 minutes for that package. Some may stay longer, some may leave earlier, but you have learned this is a good average. Much like restaurant reservations, they ask when you want to start, but they don’t kick you out after a set time. They know how long the average groups stays at one of their tables, and they plan their reservations around that. Perhaps a group will stay longer, and have after dinner drinks, desserts, coffee, etc., but that will be balanced by the group that wants to eat fast, and hurry out as they are heading to a movie after dinner.