title: What is the difference between SV_POSITION and POSITION?
body: <p>While looking at the direct3d11 tutorial sample code,
  I saw code that I could not understand in the shader code.</p>
  
  <pre><code>struct VS_INPUT
  {
      float4 Pos : POSITION;
      float4 Color : COLOR;
  };
  
  struct PS_INPUT
  {
      float4 Pos : SV_POSITION;
      float4 Color : COLOR;
  };
  </code></pre>
  
  <p>VS_INPUT use POSITION and PS_INPUT use SV_POSITION.</p>
  
  <p>What is the difference?</p>
  
  <p>why can't I use POSITION PS_INPUT?</p>