1. 1 to 6 rep
It will help you to build strength and power.
2. 6 to 12 rep
It will help you build muscle mass.
3. 13 or more rep
It will help you build endurance (stamina).
Note:-
If you doing 250 plus rep than you are working on everything such as strength,power, muscle mass and endurance.And doing reps slow will help you to build more muscle because muscle is made when body is in under tension.
Fitness and You. 