title: Visual Studio - Derived Column - Destination Editor - Why Available Input Columns include the new (derived) column?
body: <p>I've created a derived column FullName (FirstName + "  " + MiddleName + "  " + LastName) from AdventureWorks. 
  I understand that in the Mappings section we create/check the connection between the source and the destination. But the source actually does not contain the column which I ask to derive. So why does it appear in both sides of the map?
  <a href="https://i.stack.imgur.com/xh4O1.png" rel="nofollow noreferrer">Mapping Image</a>
  <a href="https://i.stack.imgur.com/oyv8P.jpg" rel="nofollow noreferrer">Derived Column Editor Image</a></p>