The following explanation is from P90X maker Beachbody for Classic, Double, Lean of P90X workout schedule
CLASSIC – Consider this the starting line to the extreme physical and mental challenge ahead. it’s time to show you’ve got. It may take some time to get use to the rigors of this program, but with patience determination, you will go the distance.
DOUBLE – The doubles plan of attack is for those who want some extra cardiovascular exercise, either for performance or weight loss. Keep in mind that P90X is stressful within the realm of its standard schedule. Those attempting Doubles should be completely injury free and have plenty of energy to add another workout to the schedule three or four days per week. The Cardio x workout is the least intense of the P90X series, but it’s still a serious calorie-burner.
LEAN – P90X lean is for those who want a more cardio – based and slightly less intensive program . Make no mistake , this is the real deal and no picnic. In fact , you may sweat more than in P90X. What you sacrifice in overall gain in strength and speed, you’ll make up fir with positive changes in lean body mass.