Understanding DFP ad tags
An ad tag tells 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.
I just upgraded to DoubleClick for Publishers. Do I have to update my ad tags?
No. You don’t have to change ad tags that were created before the upgrade. Ads will still deliver to those tags, and you'll still be able to take advantage of many of the upgrade's features such as optimization, forecasting, and the new UI.
However, if you create a new ad unit in the upgraded DFP and want to either modify existing tags or create new tags that use that ad unit, you must add your network code to those tags.