Create your own DTD plugin for DITA 1.2
IXIA CCMS Desktop includes a plugin, com.ixiasoft.dita.dtd, which is found in Repository/system/plugins. This plugin, like any standard DITA plugin, includes a file with the information needed to integrate it into the DITA standard DTDs (plugin.xml), and a catalog file (catalog.xml) that points to other DTDs and related files that you want to include as specializations in the IXIA CCMS Desktop. The plugin also includes a subfolder, dtd, which includes all of the CMS-specific DTDs.
One of these CMS-specific DTDs is IxiaDitabase.dtd. This file is essentially a copy of the DITA 1.2 ditabase DTD with some additional items that are specific to the CCMS Desktop. This is the starting point for integrating any other DTDs into the CMS, but rather than make changes to the com.ixiasoft.dita.dtd plugin, IXIASOFT strongly recommends that you create your own plugin. While this is more trouble up front than simply editing IxiaDitabase.dtd, in the long run it will save you time and really is a best practice.
If you opt to create your own DTD plugin, you should do so before you begin production work in the CCMS Desktop. If you begin production work using com.ixiasoft.dita.dtd and later want to create your own DTD plugin, you will need to change all of your topics' DOCTYPEs to reference your DTD rather than IxiaDitabase.dtd (if you want those topics to use the same specializations and constraints as new topics created going forward—which is advised, to reduce author confusion and inconsistency). This change is not trivial to make and should be avoided by as much prior planning as possible.
You now (mostly) have your own plugin. Going forward, you'll do all your integrations in your plugin instead of in com.ixiasoft.dita.dtd.
Next, you need to make some important edits to your custom topic and map shells.