When the Switch invocation code is loaded on a webpage a request is sent into the ad server, at this point a "Request" is counted against the banner intended to serve in the reporting.
Once the the third party script or image file from the selected banner has been successfully delivered to the page, a subsequent call is made to the ad server, at this point an "Impression" is recorded in the reporting.
When the UPT is in use, three additional metrics are introduced in the reporting and can be viewed using the query tool. These metrics are “Passbacks”, “Served Requests” and “Served” these ultimately change how “Requests” and “Impressions” are recorded.
“Passbacks” are again recorded against the banner and are a measure of the amount of times the UPT was served by the third party that was originally served by Switch.
“Served Requests” are recorded in the same way that “Requests” are described above.
“Served” are recorded in the same way “Impressions” are described above.
When a “Passback” is recorded against a banner neither a “Request” nor an “Impressions” is counted against that banner. After the Passback occurs a subsequent “Served Request” and “Served” is recorded against the next highest paying banner in the prioritisation. The “Requests” and “Impressions” are only recorded when the third party that monetise the opportunity (i.e does not pass back).
“Requests” = “Served Requests” - “Passbacks”
“Impressions” = “Served” - “Passbacks”