Consumer video is based on digital component YPbPr video signals using the digital range of 16-235 representing O IRE and 100 IRE, the full scale of a video signal. This is the same thing as 0% modulation and 100% modulation that you may see on currently published calibration discs rather than IRE.
When consumer video is mastered it is not uncommon for the video signal to go slightly below digital 16 or slightly above digital 235. This is directly related to using a calibration disc to set black level using a pluge pattern that provides 1 or 2 bars slightly above 16 and one bar below 16 and that bar we call below black. The difference between the below black bar and peak black is called Black To Black. Digital Video Essentials also includes WTW, White To White, video content in select patterns. Just like the below black bar this content is slightly above peak white or 235.
A properly designed display or player should be able to pass these signals and there may be some settings in the setup menu of your product to allow that such as the Sony PS3. Proper display calibration would provide a small margin of headroom to account for this. That means the calibrator will set brightness so the below black bar is slightly visible and set contrast slightly below the thresh-hold of proper peak white.
Some displays and sources will not pass BTB and WTW signals by clipping anything outside of the 16-235 range.
Digital RGB may or may not pass BTB and WTW signals and this is only a concern if your source or display uses a DVI rather than HDMI connector or if you are using a PC. The correct product design for a DVI consumer video source should provide the RGB as 16-235 which then allows digital video content above and below to pass. Like the consumer DVI source a DVI consumer video display should be designed for 16-235 RGB. The reason DVI uses RGB is because DVI is actually a PC standard and adding to the confusion of standards PC video uses the full 0-255 range of digital video. If a consumer source or display is designed for PC video 0-255 then the original 16-235 of consumer video will be scaled to 0-255 and BTB and WTW cannot pass since they exceed the available range.
Home Theater PCs of the past have had issues with BTB and WTW. Some of the currently available video cards have addressed this problem by separating consumer video sources from the desktop/PC video sources providing independent adjustments for both. A consumer HDTV example: the display is designed for 16-235 video so you would enter your video card setup menu and set your desktop for a 16-235 response either through specific presets or brightness and contrast settings and then separately set consumer video in the same manner. My video card, Nvidia 280GTX, provides video controls for the desktop and video controls along with setup options such as RGB/YPbPR and 0-255/16-235 for consumer video.
More info
DVI - video levels and color space
HDMI - video levels and color space