What does promotional inventory mean?

We make these great programs possible by connecting our customers with travel inventory that we get from our suppliers when they need our help to fill their unsold rooms, airline seats or cruise cabins. This will primarily happen during off peak times of the year or off peaks days for places like Las Vegas. Often promotional inventory for land or air/land programs will be available within 30 days of travel. For cruise lines they often give space further out rather than closer in like the airlines and hotels.