Rav Pappa said: One who threw a clod of earth at a palm tree and severed dates is liable to bring two sin-offerings: One due to severing, which is a subcategory of the primary category of reaping; and one for extracting, which is a subcategory of the primary category of threshing, as he removes something edible, the date, from its cover, its cluster. Rav Ashi said: In that case, one is exempt, since that is not the typical manner of severing, and that is not the typical manner of extracting, and one who performs a labor in an atypical manner is exempt.

They said to him: One who milks is liable for performing the prohibited labor of extracting, which is a subcategory of threshing, on Shabbat. This is because when one extracts milk from a cow it is similar to the act of threshing, where one removes the desired content from its covering.

extracting- is a subcategory of "Threshing" as one would extract the produce from the stalks...so too this is extracting the dates from that which is carrying their burden.   

and one liability due to extracting- The Ri argues on Rashi that the case is where one extracts the burden of fruit that the tree is carrying...rather, it is like the explanation of Rabbeinu Shmuel that the dates have an outer shell, and when one hits the dates then the outer shell is removed from the dates, and it is like "Threshing" where one extracts the kernels from the stalks. 

Extracting- One removes something from the place that it was grown. And, it is not attached but detached [from its source of growth]. This is a subcategory of "Threshing" as one removes the kernels from the stalks. 

Extracting- is like removing a burdern. As one removes the food from where it was contained. This is a subcategory of "Threshing." Some say that this is a subcategory of "Harvesting." This is not true as it is not attached, but just contained within the udder just like the kernels are within the stalks.