It is basically a measurement between two points in the Lab color space which is used to perform the matching calculations. According to all scientific studies, a match of 1 or less is an imperceivable difference to any human. However depending on the observer, the actual difference which can be discerned can vary significantly - and is also effected by the actual color hue which is being examined. Most observers have a hard time deciphering between colors less than 5.5 units apart.

Yellow tinted hues for example are more difficult to discern as the observer gets older. Their lense yellows with age, and tints colors to their perception making it more difficult to decipher different hues which contain yellow pigments. Diet and lifestyle also effect how crisp an observer perceives colors. Macular pigmentation is lowered in smokers and it is higher in people who consume spinach, egg yolk and orange pepper. Add into that mix that photoreceptor peak wavelengths can vary by as much as 6 nm and you will find that colors are not perceived by everyone in the same way.

The default setting in the Color Match tool is 6 - which although significant in terms of the color space difference, it is a good starting point to determine what colors match for you.