Library /sys$common/syshlp/cms$dw_help.hlb OCC, Oc ctb  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  The Classes button controls occlusion of classes. When
  this button is activated, CMS searches for and acts upon
  the first occurrence of a class found in a library (the
  first library that contains the class) in the library
  search list.

  If this button is off, CMS acts upon each occurrence of
  the class or classes found in the library search list.