Impressions per DAU (sometimes “imps/DAU”) is set by placement design, frequency caps, session count and session length. Together with eCPM it determines ad ARPDAU.
It is also the lever most likely to affect retention. Increasing interstitial frequency lifts impressions immediately; the cost, if any, shows up later in retention and payer behaviour. Report it by format, because a rewarded impression the player chose and an interstitial they did not are different product events.