As he later explains: In not so many words, this clause from the OSD means that the implementor must allow sublicensing. Microsoft doesn't allow sublicensing of their patents instanced in code and doesn't allow the implementor to use any license that demands he do. All OSI approved license demand sublicensing either explicitely or implicitely, therefore No OSI-approved license can be used. Mr. Le'Brecage then goes on to list the licenses that wouldn't be allowed to use Microsoft's Office XML specification. John is correct. Microsoft's license for its Office XML formats is not compatible with a GPL license, which means GPL software (or, to be more precise, licenses that share the sublicensing restriction previously quoted) can't use the Microsoft Office XML formats. What no one seems to bother to ask, however, is WHY this restriction exists.