DITA coding practices for DTDs rely heavily on entities to implement specialization
constrraints. As such, an understanding of entities is critical when working with
document-type shells, vocabulary modules, or constraint modules.
This topic covers general coding requirements for defining element types in both
structural and element-domain vocabulary modules. In addition, it covers how to create
@domains attribute contribution for these modules.
The vocabulary modules that define attribute domains have additional coding
requirements. The module must include a parameter entity for the new attribute, which
referenced in document-type shells, as well as a text entity that specifies the contribution
the @domains attribute for the attribute domain.