Ad request
The call a player, page or app makes when a placement is ready to show an ad.
A request carries whatever the integration knows: placement, size, environment, app bundle or page URL, device details, coarse location, identifiers and privacy signals. The quality of the request sets the ceiling on everything downstream: demand can only bid on what it was told.
Requests are counted before any decision is made, so they are the denominator for fill rate.