title: What is an "installation script" in MSI?
body: <p>Ofiicial Microsoft documentation refers some "installation script" in numerous places. Foe example, <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/aa368268%28v=vs.85%29.aspx" rel="nofollow">here</a>:</p>
  
  <blockquote>
    <p>Instead the installer writes the custom action into the installation
    script.</p>
  </blockquote>
  
  <p>I have carefully examined MSDN and a few books about MSI - and can't find clear definition what is this <code>installation script</code>, how it's related to <code>MSI</code> engine and how it's exactly</p>
  
  <blockquote>
    <p>installation script can be executed outside of the installation
    session in which it was written</p>
  </blockquote>
  
  <p>Is it some definition and documentation about this <code>installation script</code> or it is some undocumented <code>MSI</code> engine internals that are referenced just for bibliographic purpose?</p>