The @format attribute

The @format attribute identifies the format of the referenced resource.

The following values are explicitly supported:

dita

Indicates that the target is a DITA topic or an element in a DITA topic. Unless otherwise specified, when @format is set to dita, the value for the @type attribute is treated as topic.

ditamap

Indicates that the target is a DITA map. References to submaps can occur at any point in a map.

When a topic reference specifies format="ditamap", the topic reference resolves in one of the following ways:

Target of <topicref>	Resolution of <topicref>
DITA map	The hierarchy of all the topic references in the targeted map
Map branch	The hierarchy of the targeted map branch

When a topic reference targets an entire DITA map and the referenced map contains a relationship table, there are special processing implications. Because relationship tables are only valid as direct children of the DITA map, referenced relationship tables are treated as children of the referencing map.

Note (non-normative):

If a <topicref> element that references a map contains child <topicref> elements, the processing behavior regarding the child <topicref> elements is undefined.

If no value is explicitly specified for the @format attribute, the following precedence rules apply:

1. If the @format attribute is specified on a containing element within the map or within the related-links section of a topic, the value cascades from the closest containing element.
2. If a value for the @format attribute does not cascade, the processing default is used. The processing default for the @format attribute is determined by inspecting the value of the @href attribute:
   1. If the @href attribute specifies a file extension, the processing default for the @format attribute is that extension, after conversion to lower-case and with no leading period. The only exception to this is if the extension is .xml, in which case the default value for @format is dita.
   2. If there is no file extension, but the @href value is an absolute URI whose scheme is http or https, then the processing default is html.
   3. In all other cases where no file extension is available, the processing default is dita.

If the actual format of the referenced content differs from the effective value of the @format attribute, and a processor is capable of identifying such cases, it MAY recover gracefully and treat the content as its actual format. The processor MAY also issue a message.