Source file relpath_util.xsl is part of DITA For Publishers project. It is used for file path manipulation.

DITA Open Toolkit ((http://www.dita-ot.org/).) is licensed under Apache 2.0 license (http://www.apache.org/licenses/LICENSE-2.0)
DITA For Publishers project (dita4publishers.sourceforge.net) is licensed under Apache 2.0 license (http://www.apache.org/licenses/LICENSE-2.0).