"Rib" tires (e.g., Michelin XPS Ribs and BFG Commercial T/As) are made with hard rubber compounds and closed-shoulder tread ribs, and are designed for the summer heat (see first tread photo, below). They tend to wear evenly and grip well on dry roads, and they channel water pretty well in the rain. In addition, the rib tread design also runs quieter than more aggressive patterns. While the harder rubber compound and rib tread design (with very few sipes) lasts long and wears evenly during the summer heat, Rib tires perform poorly on drive wheels in sub-freezing weather, and on snow and ice.
"M/S" tires (e.g., Michelin LTX M/S2) are "all season" tires made with softer rubber compounds that are more flexible in cold weather. The tread design is a compromise between summer and true winter (snow) tread patterns; and these are popular for many vehicles where winter driving conditions do not dictate the use of "snow tires". These are also the tires that come on Eddie Bauer models and are offered as optional equipment by the Airstream factory.
The last tire photo is the Michelin LTX A/T2, an "all terrain" tire. Of these three tires, it has the softest rubber compound and most aggressive tread design. As a result, it is the one most susceptible to uneven wear; and as the tread wears down, it is also the one most likely to exhibit the rumble typical of snow tires, when run at highway speeds.
For reference, I have also included the "summary" tables from www.tirerack.com;
so that you can compare the performance differences between these three types of tires. Hopefully, this information will help members in selecting the type of tire that best fits their driving conditions and needs.
• Michelin XPS Rib Tire Performance Rating Summary
• Michelin LTX M/S2 Tire Performance Rating Summary
• Michelin LTX A/T2 Tire Performance Rating Summary
• Michelin XPS Rib tread
• Michelin LTX M/S2 tread
• Michelin LTX A/T2 tread
Note: I have no affiliation with Tire Rack. I just find that their Website is easy to navigate, and it presents information in a logical format.