The D.O.P. on a can of San Marzano tomatos stands for "Denominazione di origine protetta", in English "Protected designation of origin". According to Wikipedia...
"D.O.P. is a trademark of the legal protection of the name which is given by the European Union to those foods whose peculiar characteristics depend primarily or exclusively from the territory in which they were produced.
The geographical environment includes both natural factors ( climate , environmental characteristics) and human factors (production techniques handed down over time, craftsmanship, know-how ) that, when combined, allow for a unique product outside of a certain area production.
For a product to be PDO stages of production, processing and preparation must take place in a ' geographical area delimited. Who does PDO must adhere to strict production rules laid down in the rules of production and compliance with these rules is guaranteed by a specific control body .
D.O.P. applies to many things we use regularly for pizza, including San Marzano Tomatoes, Parmigiano-Reggiano cheese, I've heard that even recipes like Pasta Bolognese are D.O.P. Certified.
The U.S. Company that sells "San Marzano Brand" tomatoes is misleading consumers. They're not breaking any laws, but they are not selling a D.O.P. product. Now whether or not that certification is important to you is up to you. You should know that those who subscribe to the D.O.P. certification pay to do so. If the San Marzano brand are any good, they shouldn't need to fool consumers who've heard that "San Marzano's are the best!" It's like selling "Cuban Seed" cigars as a Cuban Cigar. Just not the same thing and shame on those who try it.