[Term in DART for Publishers] Zone An entity within a DART site, used to identify one or more ad slots within that DART site. Zones are used to increase the granularity of targeting within a DART site. A zone can be composed of ad slots in one or more webpages or parts of webpages. Not all DART sites contain zones.
Second-level ad unit The second 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 Publishers premium account, all of your zones will become second-level ad units upon upgrading. (Learn more about inventory hierarchy)