The "Play AVI in Dialog" menu option sets a flag
which is used by the OnHandlePlayAVIFile method 
of the HTDisplayBox component. This method is used 
to modify how clicks on links which play AVI files
are handled. In this case, the AVI files are not
put in the built-in display dialog but are 
instead sent to a TMultiMediaPlayer component
in a generic dialog. (This could be used to
direct the display to other custom components
of an application.)

(Note: Assert methods normally are used to abort
all further processing of an action, while the
Handle Methods modify the processing. These
two methods can each be overridden in code by
direct assignment for Assert types and virtual
overriding for Handle types.)
 