One subsumes the other. Rounding can be a means, a tool, one uses to make one or more numbers (that is to say, one or more quantities) compatible. For example if you were dealing with lots of something for which the standard unit of measure was in tons, and a particular lot weighed in at so many pounds and ounces, you can see that the lesser unit amount would not be significant . . . In 2 ton of crushed rock, for example, a half pound and one ounce is not significant. So you would round the insignificant amount to make it compatible with the accompanying, significant amounts. Thus 500.5025 pounds is rounded up to 500 pounds, and then again to 1/4 ton. Because you are "dealing" in tons, it will not matter that rounding might result in a ton here and a ton there being several pounds heavier or lighter. Because your rounding convention causes some lots to be slightly overestimated, and some to be slightly underestimated (because, respectively, you rounded up from .x5 and over, and down from under .x5), the net total of all such estimates will work out to be pretty much the exact amount of significant units (as in tons) that were dealt with.
