2.6.3 DTD coding requirements This section explains how to implement DTD based document-type shells, specializations, and constraints. 2.6.3.1 DTD: Overview of coding requirements DITA coding practices for DTDs rely heavily on entities to implement specialization and ►constraints◄ . As such, an understanding of entities is critical when working with DTD document-type shells, vocabulary modules, or constraint modules. 2.6.3.2 DTD: Coding requirements for document-type shells A DTD-based document-type shell is organized into sections; each section contains entity declarations that follow specific coding rules. 2.6.3.3 DTD: Coding requirements for element type declarations 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 the @domains attribute contribution for these modules. 2.6.3.4 DTD: Coding requirements for structural modules A structural vocabulary module defines a new topic or map type as a specialization of a topic or map type. 2.6.3.5 DTD: Coding requirements for element domain modules The vocabulary modules that define element domains have an additional coding requirement. The entity declaration file must include a parameter entity for each element that the domain extends. 2.6.3.6 DTD: Coding requirements for attribute domain modules The vocabulary modules that define attribute domains have additional coding requirements. The module must include a parameter entity for the new attribute, which can be referenced in document-type shells, as well as a text entity that specifies the contribution to the @domains attribute for the attribute domain. 2.6.3.7 DTD: Coding requirements for constraint modules A structural constraint module defines the constraints for a map or topic element type. A domain constraint module defines the constraints for an element or attribute domain. Parent topic: 2.6 Coding practices for DITA grammar files Previous topic: 2.6.2 Normative versions of DITA grammar files Next topic: 2.6.3.1 DTD: Overview of coding requirements