Use the Set Main File command to specify which source
  file is the main file. This designation is important
  when several different sources are displayed, because
  the Build Main File and Rebuild Main File commands
  only act upon the main file.
 
  The name of the active source window will appear as
  the default main file when the Set Main File command
  is used.
 
  Set the name of the main file from the dialog box:
 
    ┌─────────────────────────────────┐
    │ File Name: ▒▒▒▒.PAS ◄───────────│─■ Type a file name
    │                                 │   (file.PAS) here*
    │ C:\XXX ◄──current directory     │
    │                                 │        OR
    │ File List:          Drives/Dirs │
    │ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒    ▒▒▒▒▒▒▒▒▒▒▒ │ ■ Select an
    │ ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒    ▒▒▒▒▒▒▒▒▒▒▒ │   existing file
    │        ▲                        │ │ name
    │        └────────────────────────│─┘
    └─────────────────────────────────┘ ■ Choose <OK>
  *The name of the active source window will appear as the
  default.