External DTD

The benefits of using external DTDs is that they can more easily and efficiently be shared by more than one XML document, or in fact, many organizations with the need to standardize communications and data. You can write a DTD once and have multiple documents reference it. Not only does this save typing time, but it assures that as the DTD manager makes changes to the central DTD, all documents that rely on the DTD are updated in one fell swoop. (Of course, DTD changes will not necessarily be backwards compatible, so watch out!).
In order to reference an external DTD, you must change both the XML declaration and the DOCTYPE declaration.