A billboard impression only says that a screen was on. It does not say whether anyone stayed with the message. On AdVirax, the useful record is attention: how long someone faced the glass, the expression while they did, and whether a gesture followed.
Those signals are estimated on the stand and leave as anonymous summaries. The campaign report then reads dwell, emotion, and gesture in place of a single impression total, which is the same measurement the network already uses from brief to proof.