Library /sys$common/syshlp/dbg$uihelp.hlb DEBUGUI, show_line_numb  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  To hide or display source line numbers in the main window, click on
  Show Line Numbers in the File menu of that window.

  Line numbers help you identify breakpoints that are listed in the
  breakpoint view.  If you hide line numbers, more of the source code
  shows through a window of a given width.