title: What is the %% operator and ExtendedPropertySet
body: <p>All, I just found the following code when I was trying to read the stored procedure, <code>sp_addextendedproperty</code>:</p>
  
  <blockquote>
    <p>EXEC %%ExtendedPropertySet().AddValue(Name = @name, Value = @value,
    Level0type = @level0type, Level0name = @level0name, Level1type =
    @level1type, Level1name = @level1name, Level2type = @level2type,
    Level2name = @level2name)</p>
  </blockquote>
  
  <p>I have some trouble in understanding it. Please help to review my questions.</p>
  
  <ol>
  <li><p>What is the <code>%%</code> operator used for ?</p></li>
  <li><p>What is the <code>ExtendedPropertySet</code>? It seems it is not a stored procedure or function. </p></li>
  </ol>
  
  <p>Thanks.</p>