The ad type dimension is used to further describe the type of ad that the use clicked on before arriving at your web site. This dimension can be used to describe size of an ad (e.g. 300x300, or skyscraper, etc.) or the technology used in the ad (e.g. rich media, or text link, or flash banner, etc.). NOTE: an 'ad' can only have one 'ad_type' associated with it. The first ad_type set for an 'ad' will be used for all subsequent uses of the same 'ad'.