COSC (Contrôle Officiel Suisse des Chronomètres) is the Official Swiss Chronometer Testing Institute, which certifies a wristwatch for accuracy and precision, but does so according to standards established up to 1973.
There are other institutes that provide the same function, but the most important detail as it pertains to truly understanding accuracy, is that all of the truly high-end manufacturers have abandoned these certifications because their own in-house standards far and away surpass the COSC standard (and the like).
This is important for the following reason: many companies -- Rolex in particular -- not only have not abandoned the COSC, but use the fact that others have as a way to market their movements as being "better than" the rest, when in fact the opposite is (closer to being) true.
For example, the COSC standards requires a wristwatch to run within -4/+6 seconds range. Anything within that range will be awarded the certification. But many companies, like IWC (famously), Jaeger LeCoultre, etc., surpass that standard in that their movements are built to never run slow. The belief is it's ok to arrive somewhere early, but not late.
So your typical IWC standard have an average daily rate of 0/+6 seconds. This is harder to achieve, far more strict, and in it's most basic term "better" than anything COSC calls for.
What this means is that companies like IWC and JLC (and many others) refuse to submit their watches for COSC certification. Doing so, according to them, is the equivalent of "dumbing down" their watches and their capabilities.
And they're right.
The key here is that since they do not submit their watches for COSC certification, they obviously have less COSC certified watches than any other company that decides to submit all of their watches for COSC certification... such as Rolex.
Rolex submits all of their watches for COSC certification. And since achieving a -4/+6 daily rate is not that hard, all their watches get certified. So now Rolex gets to say they have "the most COSC certified watches of any other brand".
It's a very clever way to create a reality that proves nothing, but can be used to easily fool the average Joe into thinking something is better than another.
In other words, we know the average Rolex meets COSC certification, a standard the other brands have surpassed, but we don't know if Rolex watches would pass the other individual in-house standards (probably not, otherwise Rolex would have abandoned COSC as well in favor of their own in-house standards).
(I digress a bit here, but this is a tactic Rolex has mastered, and in a later article I will discuss yet another way Rolex cleverly markets itself as "better than the others", by simply using a play on words.)
And to be fair, Rolex is not the only brand to employ this type of marketing tactic. Other companies such as Omega, Breitling, TAG, Ball, ETA, etc., have also started to employ this tactic.
Whats interesting is that none of those brands, as they exist today (ignoring the illustrious past for some of them such as Omega and Heuer before the COSC was established), are considered truly high-end or "top-10" brands.
So, be smart. Don't get fooled by the marketing, and the next time someone tries to sell you on a watch because its "COSC certified" or "Chronometer certified", ask for something better!