Relational databases are great for storing structured data which conforms to a well-defined relational database schema. They are not so good at storing information that does not conform to such a schema. Since user requirements inevitably change, this means costly database upgrades.

To avoid too many such upgrades, in a larger database one often sees provisions which allow users to add relevant information which does not fit nicely in the database schema. The lowest-level extension is a free-text field in a record which allows users to add textual data. Another common extension is the introduction of user-defined value lists.

While such mechanisms are powerful, they are not standardized and do not easily allow the interchange of data. Topic Maps provide a very flexible and robust way to add arbitrary data to a relational databases at runtime. Moreover, Topic Maps come with a predefined exchange mechanism (the XML Topic Maps (XTM) interchange syntax) to allow data to be exported to XML.