Timberghost
Lots to learn on that subject. Even folks around equipment all their life have a hard time keeping up because there is just so much, and it changes pretty often.
Cat dozers have a number, from D-3 to D-11 that is the size, but only relative to each other. The letter after that is the series, and a higher letter is usually a newer model. They sometimes run out and go the other way, so it is confusing. Take your D-6 example they started as a D-6, then D-6B, D-6C, D-6D, then they skip ahead to D-6H when the high track configuration started, D-6R, and D-6T, which is the current model. Just to confuse you further, there is a D-6M, and D-6N that are a little smaller than the others, and come with 6 way blades, and now there is a D-6K, which is a little smaller, but hydrostatic drive and back to an oval track. Now that you are good and confused, add in series II, and series III in some of the models representing various upgrades, and you have the models covered. Then you get to configuration, like LGP for low ground pressure. You get longer track frames and wider pads, with a wider but shorter blade to cover the track width, XL, which is extra long, a little longer track frame makes it more stable for finish grade work, XW, extra wide, which is a little wider than standard, but not as wide as an LGP.
All that just got you a brief primer on D-6's, and there are different models for each size, from D-3 to D-11.
Each brand is different, each type of equipment is different, and the list for any of them may be as long as what I gave you for the D-6, so there is plenty to learn. My advice is to read a lot.
I think the D originally stood for diesel back in the day
