A privacy mask is a feature that allows you to obstruct a viewable part of an image so information behind the mask cannot be seen.
This feature is commonly used for masking out windows and doors in residential areas, area's of a sensitive nature or any other area's that you do not want the camera to be able to see.
When this feature is used on a PTZ camera, the PTZ camera can still be panned and zoomed as normal, every time the dome passes through the masked area the privacy mask will be applied to the exact same location every time by the dome camera.
PTZ camera's usually support multiple privacy masks for masking out different area's or providing a mask bigger than one mask setting will allow.
The privacy mask feature is also becoming common in static camera's.