The rectangle represents the confinews of the world to a painter. calling attention to the rectangle is the kind of thing the Modernist Abstract painters worked with, Mondrian for a good example.
Often finding ways to direct the eyes around the picture to bring the eye into the picture is what most classical composition is about.
This painting is a classical fruit in bowl painting with a twist of the rectangle in the composition relating to the canvas edges. The round fruits are wuite flat and the rectangle is deep and modeled.