there are cans of 1) san marzanno variety tomatoes grown in other places
2) tomatoes of some variety grown in san marzanno region, 3) and san marzanno variety tomatoes grown in san marzanno and canned there. If you get the 3rd, the real thing, it should have a DOP garauntee from the Italian GOv't on the can, the tomatoes are very meaty and sweet and make a rich sauce. Other Roma tomatoes are great too, but are not as meaty or sweet.
I use them for sauce for spaghetti. I use a great California tomato product from Muir Glen for chili, and a fine Jersey canned tom for soups, stews etc.