Beginner - You are interested in the sport and want to start playing or you play occasionally for fun. A softer ball with a colorful target would help you learn how to hit.
Intermediate - You are comfortable with a racket and play a couple of times a month. A hard, but not extremely pressurized training ball would be good to practice with since they are easier on your arm.
Advanced - You play competitively and consistently throughout the week. A more pressurized ball for competitions is right for you.