Dear Stephen,
the explanation on p. 38 of the MIR manual and especially the screenshot on p. 39 should make it clearer. :-)
[URL=http://img34.imageshack.us/i/mirdirectivityprofiles.jpg/][IMG]http://img34.imageshack.us/img34/3879/mirdirectivityprofiles.jpg[/IMG][/URL]
Kind regards
/Dietz - Vienna Symphonic Library