If  is selected, the shield in front of the image sensor will close when the camera is turned off. This prevents dust or other foreign matter collecting on the image sensor when lenses are exchanged. It may also, however, increase camera startup times.
The shield can be damaged by touching it when it is closed.
When exchanging lenses with the shield closed, be sure to insert the lens perpendicular to the mount. If inserted at an angle, lenses could contact the sensor shield and damage the shield or the image sensor.