TV/award show term. A two-dimensional wall of flats that flies and is brought in and out for each musical presentation, functioning much like an act curtain. Some shows will have as many as three: SR, Center, and SL.
In the photo above, both the SR and SL close-downs are "in". Likely, bands are being reset upstage of them.
In the photo above, both the SR and SL close-downs are "in". Likely, bands are being reset upstage of them.