The @id attribute is available for most elements. An element must have a valid value for the @id attribute before it can be referenced using a fragment identifier. The requirements for the @id attribute differ depending on whether it is used on a topic element, a map element, or an element within a topic or map.
All values for the @id attribute must be XML name tokens.
The @id attributes for topic and map elements are declared as XML attribute type ID; therefore, they must be unique with respect to other XML IDs within the XML document that contains the topic or map element. The @id attribute for most other elements within topics and maps are not declared to be XML IDs; this means that XML parsers do not require that the values of those attributes be unique. However, the DITA specification requires that all IDs be unique within the context of a topic. For this reason, tools might provide an additional layer of validation to flag violations of this rule.
Within documents that contain multiple topics, the values of the @id attribute for all non-topic elements that have the same nearest-ancestor-topic element should be unique with respect to each other. The values of the @id attribute for non-topic elements can be the same as non-topic elements with different nearest-ancestor-topic elements. Therefore, within a single DITA document that contains more than one topic, the values of the @id attribute of the non-topic elements need only to be unique within each topic.
Within a map document, the values of the @id attributes for all elements SHOULD be unique. When two elements within a map have the same value for the @id attribute, processors MUST resolve references to that ID to the first element with the given ID value in document order.
Summary of requirements for the @id attribute
|Element||XML attribute type for @id||Must be unique within||Required?|
|sub-map (elements nested within a map)||NMTOKEN||document||Usually no, with some exceptions|
|sub-topic (elements nested within a topic)||NMTOKEN||individual topic||Usually no, with some exceptions|