Byron, I am pretty sure he meant "realisticity".
The minimum radius in HO is a synthesis of what you have to push around the curves and the shape and dimensions of your overall track plan. As stated earlier, some larger steamers in HO will go around 18" curves. We all learn to live with compromises. We all learn that there is a better way to do things after we accept what "better" means to us. Often, for most of us, better usually is a combination of more/larger. We get more engines, and usually not the same kind. We develop a hankering for a larger steam or diesel monster, and there is where we realize that we shot ourselves in the foot...we didn't plan ahead, or we bought the wrong monster.
As a general guideline, subject to individual bias, the smallest practical curves in HO are accepted widely to be in the order of 18". Not a hard and fast rule, but a guideline. Mountain logging operations had tighter curves in scale and nasty track of a temporary kind.
For those with the room, and who know not to fill all their space with every conceivable track apparatus and length, but to keep tracks to an operational and effective minimum, curves between 24 and 30 inches are highly desirable, and would be called medium curves. The lower limit in that range, 24" is what Walther's states is the minimum to operate their heavyweight passenger cars. I'll bet 100 people who post here regularly would say "baloney!" They'll tell you they had to alter their cars or widen their curves before the derailments went away looking for another layout to haunt. (I have it on good authority that they are busy to this day...)
Curves in excess of 30" radius are generally considered broad curves, and if laid properly are essentially trouble free. They look the best, too, for longer rolling stock and large engines.
But, to close the loop, and this post, it is all a synthesis between what you have to run that you like, and the space you have to fashion minimum effective radius curves. It's almost a chicken and egg debate...what you buy because you gotta have it, and what curves you can squeeze into the room you have to play with.