This term is relevant primarily to sites that monetize their traffic by selling display ad inventory directly to advertisers. Remnant inventory refers to any ad units that have not been purchased by an advertiser, and this must be accounted for using another strategy (more on this below).
Ideally, a site will sell every one of its available ad impressions directly to advertisers at premium CPMs. More likely, however, is that a portion of expected ad impressions will go unsold.
For example, assume a publisher is expected to generate 5,000,000 pageviews during the upcoming month (let’s further assume there is one 728×90 leaderboard ad unit on every page). If the publisher has sold 4,000,000 of those leaderboard ad impressions to advertisers, they will have 1,000,000 remnant ad impressions that must be monetized in other ways.
There are four primary options for monetizing remnant ad inventory:
The first option is obviously less-than-ideal; it means that the publisher is wasting what could have been a very valuable ad impression. The other three options each have the potential to result in revenue for the publisher.
House ads are essentially ads that are promoting another stream of revenue for the publishers. Below is an example of a house ad on ESPN:
Without a direct-sold ad available to fill that position, the publisher instead elected to serve an ad promoting a show airing on the company’s TV station. This doens’t generate any revenue for ESPN, but can still benefit the overall company by increasing awareness of another of its products.
Options #3 (CPA-based affiliate banner ads) and #4 (CPC-based ad networks) above will likely not generate nearly as much revenue as a CPM campaign. Under both arrangements, the vast majority of ad impressions (those that aren’t clicked) are worthless. However, these remnant monetization techniques will ensure that the publisher gets at least some revenue from the ads that aren’t directly sold.
Most publishers dealing with remnant ad impressions have the goal of eliminating remnant entirely.
Using networks can lead to some complexities when attempting to sell ads directly. Specifically, advertisers may be able to get their ads on your site without buying from your sales team. If they are also running campaigns with the network you’re using to monetize remnant inventory, they may effectively circumvent your sales team and hurt your direct sales efforts.
Most ad networks will let you block specific advertisers. This feature was designed to allow publishers to prevent low quality of competitive ads from appearing on their sites, but it can be used to force advertisers to buy directly as well.
Below is a screenshot of the AdSense dashboard that lets publishers block certain advertisers: