Generally header files are not members of targets. Making an implementation file a member of a target tells Xcode to compile the file when you build the target. In your example Xcode compiles the file myAppFile.m when you build the myAppTests target but not when you build the myApp target.
When you have an application target and a unit testing target, your application’s implementation files should be members of the application target. Your unit testing classes’ implementation files should be members of the unit testing target.