In DITA, constraints refer to rules and conditions applied to DITA elements and attributes within a specialization. These rules define how elements and attributes can be used, enforcing compliance with documentation standards. Constraints are essential for guiding content creators, maintaining structure, and validating content within specialized DITA documents.

Constraints control the usage of elements and attributes in specialized DITA document types. They specify how elements and attributes can be used, where they can appear, and what conditions they must meet. Constraints serve multiple purposes, including enforcing structured content, guiding authors, and mandating document flow. For instance, a constraint can specify that within a procedure element, there must be a step element followed by an optional note element. This enforces a consistent structure for procedures across documentation.

Some examples of constraints include:

• Element Constraints: These define rules for individual elements. For example, when creating a “warning” element a constraint can be set which mandates a “warning” element must contain a “title” element followed by a “message” element, with no other elements allowed. This ensures that all warnings have a consistent structure.
• Attribute Constraints: These define rules for attributes associated with elements. For example, imagine a “severity” attribute for a “bug” element. An attribute constraint can be created which limits the “severity” attribute to only accept the values “critical,” “major,” or “minor.” This ensures that the “severity” attribute follows predefined conventions.
• Content Model Constraints: These define the structure and order of child elements within a parent element. As mentioned earlier, a content model constraint can require that within a “procedure” element, there must be at least one “step” element followed by an optional “note” element. This mandates procedures following a standardized format.

Constraints can also serve as guidance for authors, specifying what elements and attributes are available for use in a given context. They also validate content as it’s authored, alerting authors to any violations of the defined rules. This reduces errors and ensures that content meets specific criteria.