Overview of constraints

Constraint modules enable information architects to restrict the content models or attributes of DITA elements. A constraint is a simplification of an XML grammar such that any instance that conforms to the constrained grammar also will conform to the original grammar.

A constraint module can perform the following functions:

Restrict the content model for an element
Constraint modules can modify content models by removing optional elements, making optional elements required, or requiring unordered elements to occur in a specific sequence. Constraint modules cannot make required elements optional or change the order of element occurrence for ordered elements.

For example, a constraint for <topic> can require <shortdesc>, can remove <abstract>, and can require that the first child of <body> be <p>. A constraint cannot allow <shortdesc> to follow <prolog>, because the content model for <topic> requires that <shortdesc> precedes <prolog>.

Restrict the attributes that are available on an element

Constraint modules can restrict the attributes that are available on an element. They also can limit the set of permissible values for an attribute.

For example, a constraint for <note> can limit the set of allowed values for the @type attribute to "note" and "tip". It also can omit the @othertype attribute, since it is needed only when the value of the @type attribute is "other".

Restrict the elements that are available in a domain

Constraint modules can restrict the set of extension elements that are provided in a domain. They also can restrict the content models for the extension elements.

For example, a constraint on the programming domain can reduce the list of included extension elements to <codeph> and <codeblock>.

Note (non-normative):
For DITA implementations that use RNG-based grammar files, restricting the set of extension elements that are provided in a domain can be handled simply by document-type configuration.
Replace base elements with domain extensions
Constraint modules can replace base element types with the domain-provided extension elements.

For example, a constraint module can replace the <ph> element with the domain-provided elements, making <ph> unavailable.