There are three prefixes with different names that should create a mirror effect on the surface of the models.
1.... sweethome3d_window_mirror
2.... sweethome3d_window_mirror_on_hinge_
3.... sweethome3d_window_mirror_on_rail_

Note that these are basically the same.
sweethome3d_window_mirror creates a mirror effect,
_on_hinge / _on_rail attaches the mirror to a deformation.
_on_hinge / _on_rail do not change the effect of sweethome3d_window_mirror.
sweethome3d_window_mirror_on_hinge_ is a shorthand for the non-existing sweethome3d_window_mirror_and_sweethome3d_opening_on_hinge_
sweethome3d_window_mirror_on_rail_ is a shorthand for the non-existing sweethome3d_window_mirror_and_sweethome3d_opening_on_rail_

For experimenting with the mirror you don't need the deformations since deformations (duh!) don't work on a photo. It can be an easy way to create a series of photos with the mirror in different positions. For example with a bathroom cabinet mirror door that is opened in different positions for each consecutive photo. You can then use the series of rendered photo's to create an animated GIF.