msdt.adml

Group Policy template language file Reference

An .adml file contains the language-specific strings for a Group Policy template. Each .adml file pairs with a language-neutral .admx template file. The Group Policy editor reads the .adml file that matches the language of the administrator. The format is XML. One template can have many .adml files, one for each language.

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.