https://e621.net/posts/3048098?q=bedding_background

The wiki seems to imply that the bedding is NOT technically the background, but the floor is, right? Thus on_bed (tag exclusive to bedding_background), and so on?

I looked up "bedding" in forums and closest match was some tag implication for sitting_on_bed.

Seems like bedding_background is intended for pictures where the bedsheets/bedding engulfs the whole framed area. The wiki says in boldface: Do not use this tag on posts with almost the whole structure of the bed, including other elements and objects outside of it, visible. Clearly in that picture the whole structure of the bed is visible.

I would put there for background: white_background blue_background gradient_background