Pre-fetch tags request ads from the DART ad server without logging impressions at the time of the request. They're used to deliver Rich Media Flash-in-Flash creatives and In-Stream video creatives. With pre-fetch tags, impressions are logged only after the video has buffered enough to start playing.
Here's an example:
Anatomy of Pre-Fetch Tags
To get a better understanding of the pre-fetch tags, let's look at them part by part.
http://ad-region.doubleclick.net: These parameters are the same as in standard tags.
pfadx: Lets the ad server know that this is a pre-fetch (
pf) tag and an ad command for XML (
Nxxxx.site-keyname;kw=[keyword];ord=[timestamp];: These parameters are the same as in standard tags.
sz=widthxheight: Dimensions of the placement, in pixels. (The value of the
sz parameter is always
0x0 for VAST placements.)
dcmt=text/xml: MIME stands for Multipurpose Internet Mail Extensions, which tell the browser what type of content you're sending. In this case, the MIME type is
text/xml, which means that the content being sent is text in the form of XML code.