If you reference the same external file from several places in your document, you can dedicate a hotspot character format to this purpose. You assign property MacroHot to the character format, and provide a definition for the macro. For example:
[HelpStyles] ShowAvi=MacroHot
[HelpMacroStyles] ShowAvi=EF('yourname.avi')
DITA2Go provides a predefined WinHelp macro. For example:
[HelpStyles] FarTarget=MacroHot
[HelpMacroStyles] ; Topic Macro and MacroHot have a required macro content FarTarget=EF(http://www.omsys.com/)