7N is the purity of the material. The 'N' stands for Nines.
For example: 3N = 99.9% pure, 4N = 99.99% pure, 7N = 99.99999%

Cables are generally cheap within the 3-5N area but 5N onward it gets stupidly expensive due to the laboratory conditions needed to manufacture such levels of purity.
This is important because even 4N (99.99%) already has such low levels of impurities at 100 ppm (parts per million). Every time you go higher purity the decimal place gets lower.
5N is 10ppm
6N is 1ppm
7N is 0.1ppm

Any more higher beyond that is hard to achieve and is considered pure already. Imagine finding only one part of impurity in a million parts of the material.
6N grade purity can be compared to a teaspoon of sugar in 4,929 liters of water.
7N is 1/10 of that teaspoon.