It's a requirement of most broadcasters that any final deliverable conforms to broadcast video standards.  If a video signal exceeds acceptable limits of a particular specification it is said to be illegal.  

Illegal video can exist because the original source was captured incorrectly, or as part of the editing, colour grading or visual FX process digital values have been manipulated to exceed the standards, such as:

• Maximum and minimum luma values
• Maximum saturation
• Encoded composite levels
• RGB gamut

This device is normally placed at the last point in your broadcast chain or edit workflow and uses clipping techniques to bring your video output into the legal specifications required for the broadcaster you are submitting to.  Parameters can be adjusted from the front panel for various worldwide standards, and once set, the unit can work invisibly in your final output path providing the necessary correction.

Speed up your workflow with a real-time legaliser and don't rely on software plugins that don't always catch everything.  For live broadcasts these boxes are essential for correcting any real time illegal video, correcting it before transmission or encoding.  Encoders work much better when presented with legal video!