As explained above, currently, around 75% of display ad space is sold directly from publishers to advertisers. The remainder of the space is sold through ad exchanges (which facilitate the sale of ad space) or ad networks (which remunerate the publisher for being allowed to fill its ad space with ads collected from advertisers participating in the network).
As ad exchanges are online markets, it is not guaranteed that a publisher will be able to find buyers for its entire remaining inventory on one single ad exchange. It therefore makes sense for publishers to be active on two or more ad exchanges at the same time. The same is true for advertisers who can also expect to acquire the desired ad space on better conditions if they are active on more than one ad exchange. Moreover, as inventory in online advertising is dynamic, publishers are not able to sell it in its entirety on an ad exchange before it is created by the users' behaviour. Such "remnant", unpredictable ad space is also unlikely to have characteristics that would make advertisers buy it directly from publishers.
As explained above, the ensuing gap of unsold ad space is filled by ad networks. Ad networks pool advertisers on the one side and publishers on the other and therefore can provide ads for space that otherwise would have gone unsold because no advertiser was explicitly interested in this space. Advertisers may be interested to work with several ad networks at the same time because this enlarges the potential ad space available for the advertiser's ads. Publishers are also interested in working with several ad networks because these offer different remuneration schemes for participants and may also be thematically specialised which publishers may like since it helps them getting thematically diverse ads onto their web pages.
All these considerations show that it is very likely that any given website requested by a user could be filled with ads from different sources, e.g. from one or more advertisers who directly or indirectly (through an ad exchange) bought ad space or from different ad networks. At some point, it must be decided which of these ads to transmit to the user's browser. This functionality, the so-called "ad arbitration", is provided by DoubleClick's DFP. It is designed to take into account a large number of possible targeting criteria and conditions of different advertisers and ad networks to select the ad that is most beneficial for the publisher.
If most customers of DoubleClick already use one or more ad networks or
exchanges to sell or buy ad space, there is no apparent reason for them to leave these networks and exchanges and only work with DoubleClick's intermediation service rather than only adding the DoubleClick ad exchange to gain access to even more market liquidity. In practice, this would mean that publishers would see whether they could get a better price for their ad space on the DoubleClick ad exchange than they could on other ad exchanges. If this would be the case, they would sell there, if not, they would sell elsewhere. A DFP customer would therefore have an incentive to move a portion of its inventory to the Ad