Found my answer. It's a mountain biking thing.
BTW I'm looking for heavy duty due to a load issue. The highest rating I've seen for a 26 X 2.125 is 220 lb.
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Square A square-profiled tire is visually almost flat from edge to edge; the taller shoulder knobs mean you don't need to lean in much for traction in corners. "These tires can help beginners feel more confident," says Ralph Cronin, a longtime tire tech with Michelin Bicycle Support. "They can also be good in soft mud. The thing to keep in mind is that these tires tend to have lots of cornering confidence, but when they break away, they can be more abrupt."
Round A tire that's rounded from edge to edge is generally used for hardpack conditions. It requires more lean-in angle to make use of the cornering knobs and performs better at a lower tire pressure. "Usually these are more racing-oriented tread designs," says Cronin. "Rounded tires tend to break away (start sliding) a little sooner in loose or softer conditions than square-edged tires, but are often more progressive in their handling."
Medium These tires are a mix of the two shapes above. Many tires fall into this round/square combo area—they're the happy medium for the most diverse group of riders.