According to the 2020 report on Property constraints (T244043):
To make this constraint type more correctly used and easier to maintain, it is suggested that implementations recognize the definition of classes of units (e.g., the class unit of length), usually better than trying to list most or all the instances of such a class (e.g., the instances "light-year", "astronomical unit", "foot", "parsec", "ångström", "metre", "centimetre", "millimetre", and potentially hundreds of other instances).