title: What's the meaning of "-D" in other swift flags
body: <p>In Xcode's Build Settings, we always use &quot;-D XX&quot; in &quot;Other Swift Flags&quot;. Whats the purpose and the proper use of &quot;-D&quot;?</p>
  <p>And what's the result if we don't use the prefix &quot;-D&quot;</p>