A browser source is the single full-canvas, transparent OBS source a creator adds once to run Publsh; Publsh renders booked creatives inside it, so the creator never places or swaps an individual ad.
The single thing a creator adds to their broadcast software to run Publsh: one full-canvas, transparent browser source in OBS, added once. The creator never places or swaps an individual ad; Publsh renders booked creatives inside that one source. See the overlay.