I have just set up three different configurations in my project for debug preview and release. No in Xcode there is an option to specify different .xconfig files per configuration. I am now wondering in what circumstances they should/could be used and what their advantages are over just editing the build settings per configuration.
Use xcconfig files if you find yourself changing the same build settings for each project you create. Place the build settings you're changing in the xcconfig file. By using a xcconfig file you can avoid editing those build settings every time you create a project.
Having the option to use different xcconfig files for each build configuration allows you to place the debug build settings you're constantly changing in one xcconfig file and place the release build settings you're changing in another xcconfig file. Use multiple xcconfig files if the build settings you're changing have different values for debug and release builds or if you change some build settings only for debug builds and other build settings only for release builds.