XTS is provisionally licensed under the  MPL version 1.1.

The MPL applies to the source code on a file-by-file basis.

Alternative licenses may apply to parts of the XTS codebase, please check for alternative licensing in the files themselves before modifying and distributing.

This means that modifications to XTS must be contributed back to the project and that those contributions will themselves by licensed under the MPL. Specifically, by contributing code to the XTS project you grant public use of intellectual property contained in your contribution.

You can contribute either by working with the subversion repository at opentheatre.org.uk or by simply making the source code of your changes publicly available.

Details of the limitations and effects of the MPL can be found in the  MPL FAQ

Example of licensing detail in source:

The contents of this file are subject to the Mozilla Public License
Version 1.1 (the "License"); you may not use this file except in
compliance with the License. You may obtain a copy of the License at
http://www.mozilla.org/MPL/

Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the
License for the specific language governing rights and limitations
under the License.

The Original Code is XTS eXtensible Ticketing System, unreleased.

The Initial Developer of the Original Code is Alexander Macmillan.
Portions created by Alexander Macmillan are Copyright (C) 2009
Alexander Macmillan. All Rights Reserved.

Contributor(s): ______________________________________.

When contributing you should mark your contribution clearly in the code. For instance:

# Changed by Contributor Name, 2009-02-27
while working:
    break
# End changes

Note that you can skip annotating your code if you are happy that subversion provides sufficient support for identifying your contributions.