Rich black is an ink mixture of 100% black, with additional CMY ink values. This results in a darker tone than black ink alone. If you print black alone as 100% black, the resulting black may not be as dark as you might like.
We recommend using
C 60 M 40 Y 40 K 100
This will give you a deep, dark, rich black. Only use rich black for large areas of black. Not recommended for text smaller than 20pts or thin lines.