What is a .pro.user File
.pro.user files are used by Qt Creator (the IDE)
If you are building QT from the command line, you do not need to worry about this file.
You should not commit this file to Git (or other version control)
It is an XML file, and looks kind of similar to me in style to how a .vcxproj file looks (the project file type for Visual Studio)
If you keep it open in a text, you may notice that it changes while you are using Qt Creator. Qt Creator is basically saving environment state in this file.