Understanding ad tags
Ad tags tell DoubleClick for Publishers, "I belong to this ad unit. Please send me an ad that matches my targeting criteria and specified sizes." The ad server then identifies the best ad for this ad tag, logs an impression, and sends the ad to the ad tag to display on the page.
You can see the relationship between an ad unit and its ad tag in the illustration below.
Note that the ad doesn’t have to match all of the criteria in the ad tag in order to serve. For example, an ad targeted to a top-level ad unit called Sports (that includes no key-values or second-level ad unit targeting) could serve to an ad tag that includes a second-level ad unit and multiple key-values. As long as the targeting criteria in the ad tag matches the criteria for the line item (such as ad unit, key-value, or ad size), an ad will deliver.
Learn about generating ad tags.