A transaction is the buying and selling of advertising inventory (digital,… )
Ads transaction between advertisers and publishers (manually or from ad server) is done on a ad network
To fulfill an order, there is a
https://developers.google.com/publisher-ads-audits/ - Lighthouse
From https://www.iab.com/wp-content/uploads/2015/06/IAB_Digital_Simplified_Programmatic_Sept_2013.pdf