Non-guaranteed inventory refers to ad inventory sold without a publisher or network engagement towards agencies / advertisers about the volume of ad impressions that will be delivered.
Non-guaranteed inventory is remnant inventory and is sold to ad networks which bulk buy ad space and resell it with a margin on CPM or CPC basis. Retargeting networks are for instance large buyers of non-guaranteed inventory.
Non-guaranteed inventory can also sometimes be bought directly by opportunist advertisers that don’t need a specific delivery time frame.
Inventory sold on ad exchanges via real time bidding and per single impressions is also per nature a non-guaranteed inventory.
Non-guaranteed inventory has low priority in ad spaces allocation at ad server level.
Non-guaranteed inventory doesn’t require insertion order.
It has to be noticed that an ad network can buy ad space on a non-guaranteed basis from several publishers and resell it to agencies or advertisers on a guaranteed basis.
Differences between guaranteed and non-guaranteed inventory are blurring with the traction of ad exchanges.