title: What does extern mean in an obj-c header
body: <p>what does this code mean? </p>
  
  <pre><code>// myheader.h
  extern const NSUInteger Something;
  
  @interface MyObject : NSObject
  ...
  @end
  </code></pre>
  
  <p>What does extern mean here, and how can/will it be used? is it part of the object? is it global in the project? Does it matter where (in which header) is it defined? Is that a good practice?</p>