CMYK are images on paper consist of patterns of Cyan, Magenta, yellow and black dots: CMYK for printing.
Colorants and toners create different colors by blocking, or subtracting, different portions of the white light reflected by the paper that they are deposited on.
Removing light results in the colors Cyan, Magenta and yellow, called the subtractive primaries and abbreviated CMY.
The black colorant is abbreviated with the letter K, Resulting in the CMYK combination.
Changing RGB to CMYK
Black Substitution is the normal combination of CMYK colorants do not simulate the grey values well enough when an original has a shadow area that appears dark grey or black.
Substituting some black to CMY adjusts tones.
Also means more stable color on press and faster drying for press sheets.
It happens during the conversion from RGB to CMYK and in volves either under color removal (UCR) or grey component replacement (GCR).