Originally Posted by wgscott

The index shifter is quantized, and puts the derailleur in one of several (five to 12) unique positions, corresponding to the cassette.

You can also have front indexed shifting between 2 or 3 rings, and that would have 2 or 3 unique positions. However, when the angle of the chain changes, the chain will begin to scrape on the derailleur, which people like to avoid. The simplest way to avoid this is to have a continuously variable position shifting lever, so you can trim the derailleur for each unique gear. Essentially, you use your ears to fine-tune the position of the front derailleur. Until about 1987, most if not all shifters, front and back, were continuously variable or friction shifters. Some folks still regard these as superior. The problem is the more gears you have on your cassette (or freewheel), the harder it is to use friction shifting accurately. So quantized shifting in those cases is generally regarded as better or easier.