When the HELP button is pressed and the application defines a Application.HelpFile, MUI tries to obtain Notify.HelpNode from the current object (the one under the mouse pointer). If Notify.HelpNode is not defined, MUI continues asking the parent object for this attribute (usually a group, but remember: the parent of a windows root object is the window itself, the parent of a window is the application).
When an Notify.HelpNode is found, the same procedure is applied to Notify.HelpLine. Then MUI puts the application to sleep and displays the file at the position specified with Notify.HelpNode and/or Notify.HelpLine.
This behaviour allows you to define one Application.HelpFile for your application object and different help nodes and lines for your applications windows and/or gadgets.
See Implementing online help for details.