Roadblock Serves creatives from a line item (ad in DFP) together on the same page. Roadblocks in the new DFP are set up in a single line item that contains all of the creatives for the roadblock. Roadblocks are set up in a line item's settings in the 'Deliver Creatives' drop-down list. Learn more about roadblocking creatives [Term in DART for Publishers] 
Roadblock Contains two or more linked ads that serve together on the same page. Each roadblock must contain one master ad and can contain up to six companion ads.