The cut up tomatoes are cooked down into the product you want. For juice, the seeds are strained out first, and it is merely heated and canned. These are whole tomatoes, which can be cooked briefly and processed in the canner, or boiled down to about half volume for spagetti sauce. Tomato paste is reduced farther, to about 1/4 the original volume. The special Maslan pan in the photo is from Lee Valley Tools. See the links section. They are expensive, but work very well for boiling down sauces, jams and jellies, and for candy making.