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How is an impression recorded?

An impression can only be recorded when an ad has fully rendered on the page, at which point a call is made back to the adserver in the form of 'lg.php' script. If the user, for some reason or another, refreshes the page during this point of the sequence the browser (for time purposes) will attempt to reload the lg.php script and will therefore record 2 impressions to 1 request. To avoid this, the tag has embedded a random number macro so that a particular request will link up to its relevant impression, thus avoiding having more impressions than requests.