Arbortext ships with a number of "doctypes" and is capable of having other doctypes added to it. Doctypes are added via a "custom folder" process, with the custom folder location supplied via environment variable APTCUSTOM.
The main piece of a doctype is the DTD file, and a related DCF. The DCF is a configuration file which maps DTD elements and attributes to Editor behaviour (eg. what is the bold element called, what should a list insert look like).
The final piece of the puzzle is then the stylesheets. These are conceptually an extension of the DCF but add a lot of detail about what the elements and attributes should look like on screen, in print, on the web, etc.
Does that help? I have no idea what a TDT is, by the way.