straight pulling vs. rolling stock

OK, I did a search of many forums and did not find anything. This may have been answered before but, what is the difference between rolling stock/capacity and straight pulling with regard to winches? there are many winches and I know that a rolling capacity of 6,000lbs. can be doubled to 12,000lbs. with a snatch block. quote from a winch,

"Heavy duty winch gives you up to 9000 Lbs. of pulling capacity for rolling stock, 3,000 Lbs. for straight pulling."

But what is the actual difference? Thanks in advance.

Basically rolling load refers to like loading a vehicle onto a trailer or off of a trailer or a boat. The resistance is less than say being buried up to your axles in mud. Straight line pulling is what regular self recovery winches do. You want a higher straight line pull capacity than rolling load. Some 3000 pound winches have the capacity to pull a 15000 pound rolling load.

Most companies like warn and the makes rate their winches as vehicle self recovery or trailer winches. Stay away from trailer winches for off roading.

Excellent thanks.