2.3.1 ID attribute

The @id attribute assigns an identifier to DITA elements so that the elements can be referenced.

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.

Figure 1. Summary of requirements for the @id attribute

Element	XML attribute type for `@id`	Must be unique within	Required?
`<map>`	ID	document	No
`<topic>`	ID	document	Yes
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

Note: For all elements other than footnote (<fn>), the presence of a value for the @id attribute has no impact on processing. For <fn>, the presence or absence of a valid @id attribute affects how the element is processed. This is important for tools that automatically assign @id attributes to all elements.