After you have selected a topic from the list of
  additional topics, you can perform a Visit Topic or Go
  To Topic operation on that selection. If no topic is
  selected, these menu items are dimmed.

  When you perform a Visit Topic operation on a selected
  topic, the Help system displays the new topic in a
  separate window, so that you can see both old and new
  topics at the same time. This makes it possible to read
  two or more help topics simultaneously.

  See the Go To Topic menu item for the alternative to
  Visit Topic.