A payment ID system exists across all inventory to ensure a fully transparent ad supply chain. This ID system uses unique and persistent identifiers to link every impression to the entity that is paid for the impression.
The payment ID appears as a value of the partner_id field in the bid request. If you find invalid or fraudulent activity, you can use it to identify the source from the supply chain and choose to exclude it from further bidding activity.
For developer documentation, see the Real-Time Bidding (RTB) Protocol.