In general, non-guaranteed line items fill your site's unsold inventory when you haven't made any contractual impression agreements with your advertisers. Non-guaranteed line items include: network, bulk, price priority, and house. They aren't considered booked inventory by forecasting.
Non-guaranteed and house line items are considered pre-emptible because they're used to fill your site's unsold inventory. As a result, they can compete in real time on price with AdSense. AdSense ads will serve if they can pay you more than your non-guaranteed line item.