Alphanumeric classification uses combinations of letters and numbers. Subject-numeric systems use numbers and letters to represent subjects. A good example of a subject-numeric system is the Library of Congress classification system. Most subject-numeric systems require the use of an index. Phonetic systems are used primarily for the classification of names.
Phonetic systems assign numerical values to different letter sounds.
This allows file searches for names which sound similar but are spelled differently. The most popular phonetic system is the Soundex system, created by Remington Rand Office Systems Division. Types of records and the usual method of retrieval may determine the filing system. For example, a numeric system would work well for purchase orders retrieved by number. An alphabetic system would make more sense for licensing files retrieved by licensee name.