The @format attribute

The @format attribute identifies the format of the resource that is referenced. If no value is specified, but the attribute is specified on an ancestor within a map or within the related-links section, the value will cascade from the closest ancestor.

The following values for @format have special processing implications:

The destination uses DITA topic markup or markup specialized from a DITA topic. Unless otherwise specified, when @format is set to "dita", the value for the @type attribute will be treated as "topic".
The linked-to resource is a DITA map. It represents the referenced hierarchy at the current point in the referencing map. References to other maps can occur at any point in a map, but because relationship tables are only valid as children of a map, referenced relationship tables are treated as children of the referencing map.
Note: If a <topicref> element that references a map contains child <topicref> elements, the processing behavior regarding the child <topicref> elements is undefined.
(no value)
The processing default is used. The processing default for the @format attribute is determined by inspecting the value of the @href attribute. 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 format is "dita". If there is no extension, but the @href value is an absolute URI whose scheme is "http" or "https", then the processing default is "html". In all other cases where no 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, but SHOULD also issue a message.

For other formats, using the file extension without the "." character typically represents the format. For example, the following values are all possible values for @format:

The format of the linked-to resource is HTML or XHTML.
The format of the linked-to resource is PDF.
The format of the linked-to resource is a text file.