Masked objects provide a "window" into the VR world regardless of motion.
TIP: To use in this manner, window meshes should be parented to the Scene Root component so they remain static relative to the user.
This is primarily used to create windows that are static relative to the user, so the cage is seen in fixed places at all times. This can provide a constant static reference for the player regardless of motion, to reinforce the perception that the cage is the "real" world and therefore movement in the VR world is fake.
TIP: This creates the effect described in this Oculus blog post under the heading Window Into the Moving World.
This is the inverse of PORTAL Mode where masked objects show the cage, not the world.
An example window mesh, seen above, is provided in VRTunnellingPro Content > Meshes > WindowMeshes.