Help System (web edition)

F7:REF
  REF defines the current cursor position as a reference point.
     If REF is pressed another time while the cursor is indicating
     another point, the display corresponding to the reference point
     will be restored.
     If REF is pressed another time when the cursor is in the reference
     point, the current reference point is released and another may
     be selected.
     The reference point is explicitly released by PREFIX D .
     A new reference point is explicitely set by ctrl-F7 which
     is a shortcut for PREFIX D REF.