Overview of DFP Video creative sets
A creative set is a group of ads that includes a master creative and its companions. Even if there are no companions, the master ad is still called a "creative set" in DFP. You can upload multiple creative sets for each video line item. If you upload two sets for the same size, the ads will rotate.
A creative set is sent to the video player as a VAST (Video Ad Serving Template) response, which saves you from having to request the companion separately.
Video line items must have creative sets assigned to them, rather than standard creatives.