Camera lenses,why some are a single dimension, others called by 2 dimensions?

100mm,200mmetc. as opposed to 28mmx300mm ,12mmx24mm etc.

Prime vs. zoom

All lenses are two dimension - that is that they give length and width to the subject. What you are calling two dimension are really zooms that extend from 28mm - 300mm or 12mm - 24mm.

They will cover all basic lens lengths in between.

You sacrifice something with zooms, they are more convenience lenses in my estimation. Most will give you good results, but where you have moving elements, they is always a chance for something to fall out of place.

I only sold lenses for 6 years.

A 50mm, 85mm, 200mm, etc are all prime lenses, meaning their focal length is fixed at the millimeters stated. If you see a number like 12 - 24mm, or 28-105, or 70-300, that means it is a zoom lens, and has focal lengths between those two numbers. Wide angle lenses are fisheyes through about 35mm. Normal lens is around 50mm. Short telephoto 85 to the mid 100s, 200mm and above is a telephoto..

Prime lenses generally have better optics than zooms, but zooms are getting pretty good!