According to value.
The term ad valorem is derived from the Latin ad valentiam, meaning "to the value." It is commonly applied to a tax imposed on the value of property. Real property taxes that are imposed by the states, counties, and cities are the most common type of ad valorem taxes. ad valorem taxes can, however, be imposed upon PERSONAL PROPERTY. For example, a motor vehicle tax may be imposed upon personal property such as an automobile.
An article of commerce may be subjected to an ad valorem tax in proportion to its value, which is determined by assessment or appraisal.
Duties, taxes on goods imported or brought into this country from a foreign country, are either ad valorem or specific. An ad valorem duty is one in the form of a percentage on the value of the property, unlike a specific duty that is a fixed sum imposed on each article of a class, such as all Swiss wristwatches, regardless of their individual values.