Mono cartridge classification

While researching various mono cartridges I notice that Ortofon makes specific reference to the cartridge (their SPU cartridges anyway) being best suited for playback of certain mono recordings. I don't recall seeing this mentioned for other manufacturers. These references are given in the form of a number (25 or 65 is what I've seen) followed immediately by a symbol that looks like a stylized letter "n" and then the letter "m". What does this symbol/abbreviation mean? How do I know which of these a certain recording is in? and I'm assuming its not that important to worry about since I don't see it mentioned by other manufacturers - or is it?