A+ for the effort guys, some of what was posted could be correct. The doctor does measure your eyes, the focal length of the correction needed is expressed in diopters which are inverse meters of the focal length (example 1/focal length in meters).
When a bifocal contact is prescribed it is becuse a condition exists called presbyopia which can simply be put as the eye loosing it's ability to change focus. When we're young we can view a distant object and then quickly turn around and read a book 14 inches or so away from us. The eye actually manipulates the lens inside to change the power of the eye like a telescope of microscope like defined here. As we age this microscope gets stuck and just has a hard time getting to the fine focus (up close) like it used to so we need to add power to help it out. Ok lots of prep work for the answer but it's coming I promise.
So when the doctor measures for reading they will set and object lets say 30cm from the eye's and then put lenses in front of you until you can see a certain size text clearly. That power will usually range anywhere from +1.00 to +3.00 diopters, the manufacturers of some
contact lenses take advantage of a the fact that we don't always llose all of our focusing ability so they make lenses in "low add" think of them as like a +1.50 and a "high add" think of them as like a +2.50 since the eye never loses all it's ability to acomadate or change focus, if your a +2.00 the +1.50 or "low add" would work well because you would rely on the eye to contribute +0.50 to the lens. Hope that helps.