ServerDatacenter-Volume-CSVLK-6-ul-phn-rtm.xrm-ms

Windows licensing file Reference

An .xrm-ms file contains a license in XrML, the eXtensible Rights Markup Language. The content is signed XML with product license terms and activation data. The software licensing service of Windows reads these files. Product activation installs them into the license store of the system. Windows and Office ship many such license files.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.