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In metadata, the representation term code refers to, and is used in the name of, data elements whose allowable values can be represented as enumerated lists. Each enumerated value is a string that for brevity represents a specific meaning. For example, for a PersonGenderCode the allowable code valid values might be "male", "female" or "unknown". To be compliant with ISO standards a value meaning or definition must also be associated with each code.
The ISO/IEC 11179 metadata registry standard defines code as a system of valid symbols that substitute for longer values. In general, if the number of codes is small the list of valid codes and their definitions can be transmitted with a document that validates the data. Codes usually have a symbolic meaning that can be understood by a person.
One example code is a set of two letter state codes used in a US postal address. The code MN represents the state of Minnesota. Its equivalent ID using FIPS standards is the number 27. The number 27 would be classified as having a representation term of type Identifier and have the representation suffix of ID.
Another example is the three-letter international airport codes such as 'MSP' for the Minneapolis-St. Paul Airport. Although people use these codes to identify an airport, they would be classified as having a representation term of Code because they contain mnemonic information.
Sometimes identification systems are also called "codes" in common every-day language. For example people frequently refer to a location's "zip code". Because of the lack of symbolic meaning in a numeric value, zip codes would technically be classified under ISO guidelines as an identifier. For example in the NIEM and GJXDM standards a zip code is called LocationPostalCodeID. The concept is Location, the property is PostalCode and the representation term is ID.
It is sometimes difficult to determine if a data element is a code or an identifier. In general identifiers are sequential numbers used to identify a specific item in an identification scheme. If a data element has mnemonic information it is generally classified as a code.
If only two code values are needed, an indicator (Boolean true/false) representation term can be used.