Proof of play

Proof of play is the record a screen sends back saying what it showed and at what time. It exists because somebody is paying for that content to appear, and a scheduled playlist is not evidence that anything reached a display.

It matters most when the screens carry paid advertising, where the log is the basis for billing an advertiser. It matters again in regulated settings, where showing a required notice for a required period has to be demonstrable after the fact.

The quality varies. Some platforms log an item as played when the player was told to play it, others when it was rendered. Only the second tells you a dark screen would have been caught.