title: What's the _ underscore representative of in Swift References?
body: <p>In the reference section of Apple's docs there's lots of instances of this sort of thing:</p>
  
  <blockquote>
    <p><code>func runAction(_</code><em><code>action</code></em><code>: SKAction!)</code></p>
  </blockquote>
  
  <p>The Objective-C 'equivalent' of this is:</p>
  
  <blockquote>
    <p><code>- (void)runAction:(SKAction *)</code><em><code>action</code></em> </p>
  </blockquote>
  
  <p>It strikes me that it's probably important that (in the Swift reference) there's a space after the underscore and "action" is written in italics.</p>
  
  <p>But I can't figure out what this is trying to convey. So perhaps the question is... is there a reference for the conventions used in the references?</p>
  
  <p>-- here's the page I'm referencing in this reference to the underscore use:
  <a href="https://developer.apple.com/documentation/spritekit/sknode#//apple_ref/occ/instm/SKNode/runAction" rel="noreferrer">https://developer.apple.com/documentation/spritekit/sknode#//apple_ref/occ/instm/SKNode/runAction</a></p>
  
  <h2>Update</h2>
  
  <p>Swift 3 has made some changes to how function/method parameter names and argument labels are used and named. This has ramifications on this question and its answer. @Rickster does an amazing job of answering a different question about _underscores in functions that clears much of this up, here: <a href="https://stackoverflow.com/questions/39627106/why-do-i-need-underscores-in-swift">Why do I need underscores in swift?</a></p>