This is my understanding of the total ammonia vs free ammonia.
Free Ammonia is the ammonia that is toxic to the fish and other inhabtants of the tank.
Total Ammonia is the level of ammonia in your tank that is toxic and non toxic. For example some additives like AminoLock will convert the toxic ammonia into ammonium (i think thats the correct word) and then its not as toxic to the tank. Total ammonia would detect both toxic and non toxic levels.