Used when building Xcode (iOS) or Eclipse (Android) projects.
On iOS, this setting will append an existing Xcode project. Existing Xcode project setting changes will be preserved. With the IL2CPP scripting backend, this setting will also allow incremental builds of the generated C++ code to work in Xcode. Appending for iOS is only supported on OSX and Windows.
On Android, this setting will create a new Android Studio project. Existing project changes will be discarded.
Additional resources: BuildPipeline.BuildPlayer.