In DocBook, you may assign an ID to virtually every element used. Although this might be useful in some cases, it also adds a lot of overhead to document, as those IDs need to be managed and handled. Therefore, we use IDs only for a few elements, particularly for the structural elements like book, chapter, appendix, and section.

IDs have to be unique within a document. Note that the term “document” may refer to more than just one file, for example when you include many XML files in one “master” file.

ID names may not contain spaces or special characters. They may (and should), however, contain dashes, to make them more readable.