For regulations surrounding backgrounds, see: Ranking Criteria 6w3f14
Background (BG) images and videos can be added to beatmaps. When written only as "background" or "bg", it usually refers to the background image.
Backgrounds are displayed on beatmap cards, on the song selection screen, and during gameplay. If difficulties on a beatmap have different backgrounds, the difficulty with the lowest BeatmapID is selected for its beatmap card and beatmap carousel icons.