One pound to eighteen pounds of laundry. Timeless, really.
As with any washer, there are things that the 1-18 handles really well like towels, mixed cotton loads, etc. and some loads it doesn't do quite as stellar of a job with. Large loads of blue jeans, bulky comforters and the like won't turn over as rapidly. Your mileage may vary.
I love mine for super-sized towel loads, nothing washes towels like the plunging Jet-Action of a Frigidaire and rinsing is excellent. The fibers of towels in particular seem to fluff up, even old towels have been revived to a degree with that wash action. Less wear it seems, too. I have a set of microfiber fleece sheets and a couple of throws that it does a great job with, never a tangle and always well rinsed.