What's the difference between a chronometer and a superlative chronometer?

To achieve certification as an official chronometer from the COSC, a movement has to pass tests in five positions at three different temperatures, 8°,23° and 38° Celsius, over a 15-day period, after which the average daily rate must fall between - 4 and + 6 seconds The most notable deficiency of a chronometer certificate is that it only tests the movement, not the finished watch The certification is open to any watch brand, but Omega is currently the only brand to offer it, and only for its Co-Axial Master Chronometer models.

The rated water resistance of the watch according is also tested in water METAS ultimately require the watch to have an average daily rate between 0 and + 5 seconds Its "Superlative Chronometer" label requires an average daily rate between -2 and + 2 seconds per day when cased, tested in seven static positions and on a rotating rack simulating real life wear, performed over a period of 24 hours.

Rolex make no specific mention of any magnetism requirements that their movements undergo, and the testing process is somewhat of a secret, but they have been producing their Milgauss watch, which is resistant to 1,000 gauss, since 1956, so we can be sure that their standards are strict. . Source