The Known Libraries list box contains a list of all
  the CMS libraries you previously specified (if any)
  through the Known Libraries... submenu under the
  Options menu. The Known Libraries list box can
  contain ten library names.

  If you click on a library in the list box, the library
  is placed in the Library Directory field.  Double click
  on the library in the list box to place it in the
  Library Directory field and open it.

  If you press the SHIFT key, then click on the library
  in the list box, the library is appended to any libraries
  in the Library Directory field.  Press the SHIFT key,
  then double click on the library in the list box to
  append the library to any libraries in the Library
  Directory field and open it.