In the previous releases of DITA Open Toolkit, a simple XML catalog resolver is enabled. You do not need to update the reference to dtd in DITA files when the file paths are changed; however, this simple implementation cannot be redistributed because it does not support standard XML catalogs.
In DITA OT 1.3, a standard XML catalog resolver is enabled so that the reference to dtd in DITA files does not need to be updated each time when you change the file paths on your workstation or use another workstation.
With this enhanced feature, when a developer makes a new specialization, the developer only needs to update the mapping between the new dtd file's system id (location relative to the catalog file) and public id (the id assigned by the developer in the head of the DITA or xml file which identifies the corresponding dtd file) in the catalog file (catalog-dita_template.xml), for example, <public publicId="-//IBM//DTD DITA ABC//EN" uri="dtd/abc.dtd"></public>.
This enhanced feature does not change the normal behavior of the toolkit.