When the HELP button is pressed and the application defines a MUIA_Application_HelpFile, MUI tries to obtain MUIA_HelpNode from the current object (the one under the mouse pointer). If MUIA_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 a non NULL MUIA_HelpNode is found, the same procedure is applied to MUIA_HelpLIne. Then MUI puts the application to sleep and displays the file at the position specified with MUIA_HelpNode and/or MUIA_HelpLine.
This behaviour allows you to define one MUIA_Application_HelpFile for your application object and different help nodes and lines for your applications windows and/or gadgets.