"Names" have a lot to do with marketing. And while the USA influenced a lot of Yamaha marketing, and while most of the information available today is for US models, other countries received different models and different variations of the same models. It's impossible to list "positives and negatives" for entire product lines many of which spanned many decades... it would be much better to define a "displacement" you're interested in and a general year range... and if it's not a US model, you'll need to be mindful of that.
A brief history of model designations to possibly clear some confusion... Before 1974, Yamaha models were loosely based on a letter and number code. In the 1960's they used a prefix letter to designate the frame type... "Y" for a conventional "fuel tank between the knees" frame, and "M" for a "fuel tank under the seat" step-through frame. The next letter generally determined the approximate engine displacement... F ~ 50cc... J ~ 60cc... G ~ 80cc... H ~90cc... L ~100cc... A ~ 125cc... C ~ 175cc... D ~ 250cc... M ~ 300cc... R ~ 350cc. Finally there was often a suffix letter or two... "S" was used for a "street" or sometimes "sport" model, designed for pavement use... "T" was used for a street legal version that was also suitable for "trail" use, these models were the precursor to the "Enduro". (A "K" suffix was also briefly used starting in 1964 to designate models with autolube... this ended when autolube became standard on most models) A number was used to designate the "version" of the model... (model years weren't as common in the 1960's as they are now... a single model might have been sold for several years without any significant changes)
By 1968, Yamaha had pretty much dropped the Y and M frame prefixes since the early small displacement step-through models which helped launch motorcycling in the US were no longer common here. So the (arguably) first Yamaha Enduro was designated "DT1" in Yama-speak... or spelled out... ~250cc, trail model, version 1. The AT, CT, etc. models soon followed.
In 1974, nearly the entire Yamaha Enduro line received a major redesign. Along with this update, Yamaha decided to pretty much drop the old letter-number code system and capitalize on the popularity of the model that started it all, the venerable DT1. Pretty much every "Enduro" model became a "DT" followed by the displacement... so for example, in 1974 the AT, CT, DT, and RT models became the DT125, DT175, DT250, and DT360 respectively.
One positive for Yamaha is they tended to "recycle" many parts for decades. There is a lot of interchangeability. 125's and 175's were in production for many decades and sold all over the world after they were discontinued in the US in 1981... Outside the US, the smaller 50cc and 80cc bikes (which were not very popular here after the 1960's) also stayed in production virtually forever... and often with the same parts... so they tend to be easier to get parts for like the 125's and 175's (especially outside the US).
One outlier on the name is the GT Enduro series... It was introduced in the USA as the "GT1" in 1973 (~80cc, trail model, version 1). In 1974, it became the "GT80" here. It was the only Enduro model I can think of that did not become a "DT" in 1974.... though in the 1980's it did eventually become a DT80 here). The US version of this model had very minor changes so tons of parts interchange here.
And that underscores the need to research exactly what you're looking at... other countries got very different versions of the DT80 from what we got in the US. DT50's are in the same class... the US only got the water cooled "17W" variation of the DT50 in 1988-90... other countries got many variations of the DT50 and DT80, including early air cooled versions and even some powered by Italian Minarelli engines.
US did not get an "MR" model as far as I know. It looks like it was a 50cc bike produced around 1980 with enduro styling?
"TY" was Yamaha's line of "observed trials" bikes here in the USA.
americanmotorcyclist.com/racing/off-road/observed-trials/ Not street legal here, designed for trials competition. Was not the most popular here, we didn't get as many models for as many years as other countries. They are fun and make a decent off road only bike (and some have registered them to be street legal here, allowed in certain states) Parts might be harder to find for some models.
The "IT" models were another competition bike. This time for what was called "ISDT" style competition at the time. (Now ISDE). They are heavily based on the "YZ" motocross line of bikes with changes to tuning (suspension, gearing, noise level, token lighting, etc) to make them more suitable for ISDT types of competition. Most models were full on competition bikes. Not street legal in the US, though some have been registered for the street.
Which brings me to my last point... Yamaha designs of the early Enduros came from racing and racers... For example, the DNA of the YZ and IT race bikes is clearly present in the later monoshock DT models. This also means more parts interchange than might be expected.
Not sure that answers your question very well.
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