Definition of "backing"

In general, the locomotives are painted dark green; the numberplates are brass, with a red backing, and are usually carried on the tank side.

1959 October, Norman Jones, “The Manchester Ship Canal Railway”, in Trains Illustrated, page 495

The simplest and cheapest answer is to introduce a scenic backing of some kind, a backdrop, a drape, some flats to restrict the shot and obscure whatever is outside.

2013, Gerald Millerson, TV Scenic Design, page 134