In reply to Jonny2vests: Actually, in this case highclimber is right.
I think the best description of the practical significance of the numbers is at
http://www.highinfatuation.com/blog/straight-from-the-mammoths-mouth-things... . Here are two quotes:
"One of the big differences between ropes will be how climbers wear it out—if they wear it out from falling a lot, or if they wear it out from abrasion on rough rock. Although there are other elements in play that we can use to affect these properties, one of the differences in construction we use that reflects these design priorities is the relative thicknesses of the core vs the sheath—we can make two 10mm ropes, one having a thin core and thick sheath, and the other having a thick core and thin sheath. All other things being equal, the rope with the thick core and thin sheath will hold more falls before going flat, but wear faster from abrasion; and vice versa..."
"...People who are comparing two ropes of similar diameters can usually see this in the test results—Mammut publishes the % of each ropes weight that is sheath so that people can judge for themselves what rope they will be happiest with. If you fall a lot, choose a rope with a high fall rating; if you don’t fall that much then choose a ropes with a thicker sheath (and if the manufacturer doesn’t publish that info call them and ask for it!). If you climb both abrasive rock and you fall alot, then think about how you wore out your last rope—if it went flat 10 or 15 feet from the ends, then get the rope with the high fall rating for the size and if the rope just fuzzed up to the point it felt sketchy or fat or lost its dry treatment, then concentrate on a rope with a thick sheath and a compact weave."