To reuse specific DocBook modules, reference them from within your own DTD driver file, making sure to use the preferred formal public identifiers for them. Though you could also reference them from an internal subset, if you are supplying a significant amount of markup structure you're likely to want to use the same DTD with many documents, and will want to store them in their own file.
For example, here is a new DTD for software product ``release letters'' that uses the whole DocBook information pool:
<!ENTITY % infopool PUBLIC
"-//Davenport//ELEMENTS DocBook Information Pool V2.3//EN">
%infopool;
<!ELEMENT ReleaseLetter - - (Title, ProductNumber, RevHistory,
(%component.mix;)*, LetterSect*)>
<!ATTLIST ReleaseLetter
%common.attrib;
Status (Draft|Shipped) #IMPLIED
>
<!ELEMENT LetterSect - - (Title, TitleAbbrev?, (%component.mix;)*)>
<!ATTLIST LetterSect %common.attrib;>
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