|DITA map files / DITA map vocabulary|
The navref element is a specialist element used when constructing dynamically-merged modular Help systems.
Some Help system output formats, including Eclipse Help and Microsoft HTML Help, support runtime merging of modular Help files, where a master Help system incorporates slave Help system modules dynamically when the user opens the Help.
The navref element provides for run-time modular navigation structures. In other words, the target defined by navref is resolved not when the parent ditamap is being built, but when the resultant deliverable is being rendered. It is a fairly obscure element, and is not commonly used.
You may choose to use navref as a placeholder when you are building a document suite, where a constituent ditamap is not yet available. Even though the navref href is not available, the ditamap will still build. When the map referenced by the navref is available, the navref should be replaced by a normal topicref.