VAST (Video Ad Serving Template) is the IAB standard XML schema that governs how video ads are requested, served, and tracked. A VAST tag is a URL that, when called by a video player, returns an XML response containing the ad creative location, ad duration, tracking event URLs (impression, quartile completions, complete), and click-through URL. The video player reads the VAST response and knows exactly what video to play, for how long, and what events to fire. Without VAST, every publisher and ad server would need custom integration code — VAST creates a universal standard that any compliant video player can interpret from any ad server.
For CTV, VAST 4.x is the current standard — it supports high-bitrate video creative, verification signals (for IAS/DoubleVerify integration), and better pod ad management. VAST is distinct from VPAID (an older interactive ad format), which is largely deprecated in CTV because JavaScript-dependent VPAID does not run reliably on TV app environments. In India, major CTV publishers accept VAST 3.0 and VAST 4.x tags from advertisers. When trafficking a CTV campaign, your ad ops team will need to provide a VAST tag to the publisher or DSP — the creative must be hosted at a reachable URL, meet the publisher's bitrate and file size specs, and the VAST tag must fire tracking beacons correctly. Always test VAST tags in a VAST inspector tool before campaign launch.
Full guide
For a complete explanation, read: VAST tags, deal IDs, and ad trafficking for CTV campaigns: a practical guide