Page "YCbCr" Paragraph 16

Since the equations defining YCbCr are formed in a way that rotates the entire nominal RGB color cube and scales it to fit within a ( larger ) YCbCr color cube, there are some points within the YCbCr color cube that cannot be represented in the corresponding RGB domain ( at least not within the nominal RGB range ).

This causes some difficulty in determining how to correctly interpret and display some YCbCr signals.

These out-of-range YCbCr values are used by xvYCC to encode colors outside the BT. 709 gamut.