Carburetor jet sizes are measured in hundredths of a millimeter. For example, a 105 jet has an opening that is 1.05 mm in diameter and a 120 jet has a size of 1.20 mm.
To obtain maximum performance from your scooter, the carburetor must supply the correct fuel/air mixture all the way though the full range of throttle openings. What controls how well the scooter runs when the throttle is most of the way open is the carburetor main jet. This is basically a hole, and the bigger the hole, the more fuel can flow through it. How big the main jet needs to be depends on the size of the carburetor and how much air is flowing.
Since the air is less dense at high elevations and at high temperatures, you may need a smaller hole (less fuel) at 95 degrees in the high dessert than you do at sea level and freezing temperatures.