Impressions expiration

All creative types use delayed impressions . This means that impressions aren't counted until the creative renders and sends an impression request back to Google. Because of this secondary request, the Bids metric total can be greater than the billable Impressions value for these creative types.

Expiration windows

The expiration window for impressions-based billing events varies by inventory type. The expiration window of the request in seconds is sent in the Imp.exp field in OpenRTB, and adslot.impression_expiration_seconds in the Authorized Buyers protocol.