title: What is target membership in Xcode 4
body: <p>I am searching for an example for target membership in Xcode. I am a bit confused about what it means exactly. 
    For example, I have two different target membership.
    myApp and myAppTests.
    What does it mean if I specify a file (say myAppFile.h) to be part of myAppTests but not myApp?
    What happens exactly when my app build and run?</p>
  
  <p>Thanks</p>