The fist three numbers is the length of the belt, the next two numbers is the width, and the last two numbers is the angle (V) of the inner part of the belt.
The GY6 engine comes in two engine case lengths, a long block (measurement from the top engine mount to the rear shock mount) and a short block
The long block usually takes a size 842-20-30 (150cc) belt and the short block takes a size 743-20-30 or 835-20-30 (150cc) belt
V-Belt are made of High Quality Rubber and Armed with Kevlar cords, Kevlar Belts last 2-3X Longer then average belts