[Term in DART for Publishers] Site DART sites comprise a network and organize inventory for both targeting and reporting purposes. You can group one or more ad slots in DART sites. A DART site can be composed of ad slots in a webpage, part of a webpage, or across multiple webpages, and is not necessarily a representation of an actual website URL. You can further organize your inventory by dividing DART sites into subsections called zones.
First-level ad unit The first level of your inventory's hierarchy. You can find your inventory on the "Inventory" tab. If you're upgrading from DART for Publishers to a DoubleClick for Publisher premium account, all of your sites will become top-level ad units upon upgrading. (Learn more about inventory hierarchy)