Tracking an ad served in VAST format is done using a collection of VAST tracking elements at different levels in the VAST response. These tracking elements each contain a URI to a resource file or location on a server defined by the ad server that sent the VAST response. The resource file is usually (but not always) a 1x1 transparent pixel image (i.e. tracking pixel) that when called, records an event that is specific to that tracking pixel.
The Creatives (plural) element is a container for one or more Creative (singular) element used to provide creative files for the ad. For an InLine ad, the Creatives element nests all the files necessary for ad display, playback, and tracking.