Capability indices are ratios of the process spread and specification spread. They are unitless values so that you can use them to compare the capability of different processes. Some capability indices consider the process mean or target. Many practitioners consider 1.33 to be a minimum acceptable value for capability indices; and most practitioners believe a value less than 1 is not acceptable.
A company manufactures stainless steel bowls that are 12 cm in diameter. The customer requirements are that the bowls can be no smaller than 11 cm and no larger than 13 cm.