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

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

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