The format attribute

The format attribute identifies the format of the resource being cross referenced. The processing default for format is "dita". 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 inherit from the closest ancestor.

Using the format attribute

Possible values for this attribute include:

dita
The destination uses DITA topic markup, or markup specialized from a DITA topic. Unless otherwise specified, the corresponding default type will be treated as "topic."
html
The format of the linked-to resource is HTML or XHTML.
pdf
The format of the linked-to resource is PDF (opens a new window).
ditamap
The linked-to resource is a DITA map. It represents a referenced hierarchy at a position within referencing hierarchy, and a referenced relationship table included outside the referencing hierarchy
(no value)
Processors default to "dita"

For other formats, you can use the file extension without the "." (for example, in a link to file "readme.txt", use "txt" as the value).