DocHeb is correct. "Light Duty" flail mowers are used for finish mowing; you may see them being used on fairways and school grounds. They won't throw stuff like a fixed blade and give a nice cut. You'll also see the "Heavy Duty" flails cutting (hammering really is the correct term) quite thick brush along the highway. Again, the flail won't throw like a brush hog would. In both cases, whether the light duty for finish mowing, or the heavy duty for brush, the flail is better (not perfect) for use in places where you don't want to take the chance of launching artillery at nearby bystanders, in contrast to the sword or meat cleaver like blades on the finish mower or brush hog. Downside in both cases is that LD flail costs more than fixed blade finish mower and HD flail costs more than brush hog.
Make sure you accurately define the application as well as which class of flail is being discussed.