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
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.