A backplate lets you load a high resolution 2D image in the background of a project. Once loaded, the backplate image remains in the background no matter where you move the camera.
3D models in the scene retain the lighting specified by an HDR environment image, without influence from the backplate image. Backplate images do not emit light in a scene, do not cast shadows, and do not reflect in shiny surfaces. They enhance the background.
If you edit the source image for the loaded backplate, the backplate in the scene automatically updates if you selected Auto-Update in the Scenes 
tab of the Palette.