Choosing the overview causes a Help window to open,
  displaying the overview topic. All the help for the
  application is available from this topic. You make an
  appropriate selection from the list of additional
  topics.

  Not all the topic titles are displayed in the
  additional topics lists---the lists would get too
  long. If you do not see what you are looking for, pick
  something close, and then scan the additional topics
  list of the new topic. Alternatively, do a title search
  by selecting Title... from the Search menu.

  For example, in the overview topic for the Calendar
  application, only the menu names are displayed in the
  list, not the individual menu items. So here, you
  would choose the menu name first, and then select the
  menu item you are looking for from the next list.