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COSC is, basically, -4/+6 as Stonehenge said. There are no other widely accepted independent certifications of a chronometer as far as I’m aware. Omega’s METAS “master chronometer” certification is their own definition (adding magnetic resistance), as is Rolex’s “superlative chronometer” standard of -2/+2.
I think this (-2/+6) is for the older in house movement, the new, smaller one for BB58 is supposed to be -2/+4. I’ve read it somewhere where they compared it to Rolex which is -2/+2 saying that every guaranteed second means a lot more investment.