DITA enables modular document development. Not only can topics be re-used, but any group of topics or elements can be treated as a modular document component.

Modularity is the technique of building large complex things in smaller, self-contained pieces. Modular furniture is a good example of modularity in practice. Instead of being one large piece, a modular couch may be four pieces that can be arranged in different ways to form different couch configurations.

Creation of a manual can also be modularised. Instead of being constructed as one piece, a large manual may be designed as a collection of different modules, and those modules can be arranged into different configurations to create different manuals. Modular manuals are easier to maintain, and can be produced more efficiently. A ditamap referencing a group of topics can be embedded within another ditamap, just as a set of elements within a single topic can be re-used in many different topics.

XML technologies (such as XPath) make modularity technically easy to implement. DITA can take advantage of such complementary XML technologies.