Retain pure black

Often the proofer's inks are not the precise hues of cyan, magenta, yellow and black. This will prompt the ICC engine to emulate a neutral black by mixing other colours into it.

While this will give you the correct colourimetric value for black, it makes small text elements appear incorect when viewed up close.

Retain pure black, will make the ICC engine use the proofer's black ink only for exactly black elements such as text. This makes the text look sharper and appear more correct.