XML DTD Modules Contents 5.1. Implementing Document Model Modules in the DTD 5.1.1. This section is normative. Implementing Document Model Modules in the DTD Partitioning of the document model occurs at the abstract module level. In this specification, each module is an atomic unit and may be represented as a separate file entity.
Modules can even contain modules, creating a hierarchical structure mirroring the document model.
Note that modules are not always implemented as separate file entities, and modular DTDs can be easily normalized into single file versions for more efficient distribution over the Web. Higher-level modules are sometimes delivered as individual file entities to facilitate portability and reusability. Complex document models often resort to extensive parameterization of abstract modules to facilitate understanding, markup reuse, extensibility, and maintenance. In light of this, a compromise must be made between markup flexibility, complexity of the DTD, and ease of maintainability. These include declarations for common names, attributes, parameter and character entities.