Unnamed background characters that are not part of an image's focus, usually to bring life to a scene and make spaces look less empty. They may be drawn at considerable distance from the viewer (post #1648083), or the artist may use other means to establish that they are not a significant part of the image (post #2370885). In general, characters described by this tag don't count towards tags like multiple girls and multiple boys.
If there are large number of background people in the image, the tag crowd is applicable.
