Intra- vs. Inter-class unit conversion

Notice also the titles: "Set up product-specific conversion rules for units of measure in the same unit class" and "Set up product-specific conversion rules for units of measure across unit classes".

For example, if you're setting units in the same class and you convert from meters (m), target units are also units of length, such as feet (ft).
If you use conversion "across unit classes", you can set up any kind of conversion, such as from meters to pieces, from meters to kilograms and so on.