The principal acts of labor (prohibited on the Sabbath) are forty less one--viz.: Sowing, ploughing, reaping, binding into sheaves, threshing, winnowing, fruit-cleaning, grinding, sifting, kneading, baking, wool-shearing, bleaching, combing, dyeing, spinning, warping, making two spindle-trees, weaving two threads, separating two threads (in the warp), tying a knot, untying a knot, sewing on with two stitches, tearing in order to sew together with two stitches, hunting deer, slaughtering the same, skinning them, salting them, preparing the hide, scraping the hair off, cutting it, writing two (single) letters (characters), erasing in order to write two letters, building, demolishing (in order to rebuild), kindling, extinguishing (fire), hammering, transferring from one place into another. These are the principal acts of labor--forty less one.
The reason for the list in the first place is that each distinct violation of the Sabbath law requires an offering, so you'd need to know what the major distinct acts are. Each category includes not just what the label says but anything that involves the same kind of work as the activity in the label. It's clear from the tractate that one of the reasons for interpreting the list in terms of labor done to build the Tabernacle is to explain why some of these particular actions are treated as distinct from each other, and another, of course, is to assist in determining whether a given action falls into one of these categories -- the closer it is to Tabernacle-building work, the more certain you can be that it is a violation, and the farther it is from such work, the more room for difference of opinion among rabbis. So, for instance, lots of scraping-type activities can get counted as being the same kind of work as "scraping the hair off"; sweeping and polishing a floor, however, ends up being more firmly in the category than a lot of other kinds of scratching and scraping, because it's very much the sort of thing you'd do in constructing a Tabernacle. But there can be complications. I found the following anecdote somewhat amusing:
"Slaughtering." Under which category? Rabh said "dyeing," and Samuel said "taking life." Said Rabh: "I said something which may seem absurd, and so as to prevent future generations from deriding me I will give a reason for what I said: Butchers are in the habit of coloring the throat of the carcasses with blood, in order that people may see (that the meat is still fresh) and be induced to buy."
Which goes to show that there may be underlying reasons for unexpected classifications.
