Sorry. IFFYM is IF IT FITS YOUR MACROS or some are now calling it DCA (discretionary calorie allowance). Basically you set your calorie intake for your goals, get the optimal macros (protein/fat/fiber and maybe carbs) for muscle growth and/or retention and if you like you can use some discretionary calories for a treat.
Here are some links....if you have any question you could PM me.
http://forum.bodybuilding.com/showth...hp?t=133634471
http://forum.bodybuilding.com/showth...hp?t=156380183