Windows.UI.SettingsAppThreshold.en-US.pri

Package Resource Index Reference

A .pri file is a Package Resource Index on Windows. The file contains a binary index of application resources, for example strings and image paths. The resource loader reads the index and finds the correct resource for the language, scale, and theme of the user. The MakePri tool creates the file during the application build. Windows apps and system components ship .pri files next to their binaries.

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.