Under Color Removal or Under Color Reduction is used when separating LAB or RGB to CMYK. The goal of under color reduction is to reduce the CMY parts in black pixels in order to reduce the overall color coverage.
Colours from only one or two primary colours, e.g. pure cyan or pure red from magenta and yellow, are not affected by UCR. The UCR process is only used for colours consisting of all three chromatic colours.
The method is permanently implemented for separations via a CMYK ICC profile and cannot be changed. The limitation of the maximum ink application to 330 or 300% is a result of UCR, because higher ink applications would on the one hand prolong the drying times considerably, and very thick layers of ink can become cracked, e.g. when folding paper.
A second process is GCR – Gray Component Replacement.