Click this button to move the highlighted file to the top of the left-hand
 list box.  This top-most source specifies the source for the target of
 the complete build.

 When the file is moved to the top of the left-hand list box, it's name
 (without file extension) is also moved to the Main Module field.
 The Main Module field indicates the main module to be included from the
 object library during the link operation.