Ad inventory refers traditionaly to the quantity of available ad placements for selling to advertisers for a given period. Ad inventory is commonly expressed in ad impressions per month. Big publishers and ad networks speak of several billion of ad impressions per month.
For online and mobile advertising, the ad inventory notion is far more complex than for traditional media because of the targeting criteria variety. A single ad placement may be sold with different targeting options and different ways of selling.
Ad inventory can be classified by nature (quality of placement or quality of targeting):
Ad inventory = premium inventory + ordinary inventory
Ad inventory can also be classified by the sale process or result:
With RTB through ad exchanges and wholesales of non-guaranteed display to ad networks, unsold inventory is most often rare and negligible. However, to protect the value of their inventory some publishers choose not to sell ad impressions below a CPM threshold. In this case, unsold inventory is used for house ads or bartering.
Thus, ad inventory is made up essentially of premium inventory ideally sold on a guaranteed basis and remnant inventory. From case to case, respective shares of inventory varies by volume and value.
Distinction between premium inventory and remnant is complex. A priori, premium inventory is made up of best ad placements and most sought targeting criteria and is sold through direct chanels (in house sales team).
However, if premium inventory stays unsold, it can be sold through indirect channels and become remnant inventory.
With behavioral targeting, data vendors and ad exchanges, frontiers between premium inventory and remnant inventory are blurring.
For selling future inventory to advertisers and agencies, publishers and ad networks have to do ad inventory forecasting.