→ The Unofficial Guide to xcconfig files
∞One of the least documented aspects of the configuration process are xcconfig files. As of this writing there seem to be no documents provided by Apple that explain how to use xcconfigs or why they exist. A xcconfig file is used as a supplemental file to a specific build configuration. A build configuration can have an associated xcconfig file, this allows for additional changes to the target's build settings from outside the Xcode project editor.