@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 is not declared to be XML ID; this means that XML parsers do not require that the values
of those @id 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 need to
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.
@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 |
<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.