Average concurrents is the average audience present while an ad was actually on screen, derived as viewer-seconds divided by on-screen seconds, and always measured within a single platform.
The average audience present while an ad was actually on screen, designed to be derived as viewer-seconds divided by on-screen seconds. It is what separates four hours in front of a small room from twenty minutes in front of a huge one, even when the totals match.
It is measured within a single platform and never blended across platforms, since a creator's audiences on different platforms behave differently.