The numbers we see on a magnified optic; whether it is variable or fixed, are the specifications of magnification and size of the objective lens. For example:
With the numbers listed above we know that the optic is a variable power scope that offers a three times normal magnification at its lowest setting and nine times normal magnification at its highest setting but can also be viewed anywhere in between.
The 'forty' at the end is giving the measurement of the diameter of the objective lens in millimeters (mm). This holds true with any set of numbers:
When you have a fixed power optic you will only be able to view a single magnification. The numbers will give you the magnification first followed by the diameter of the objective lens (like above on variable optics) for example:
Now when you are optic shopping you will be able to understand what that jumble of numbers and letters mean and make an informed decision