COSC, the Swiss body that issues the certification, states: "A chronometer is a high-precision timekeeping instrument, set to operate with great precision under various conditions of use." It adds that a chronometer "is certified by a neutral official body, such as the COSC in Switzerland, which carries out rigorous testing of the watch or movement and issues an official certification."
COSC's own published criteria: a testing period of 15 days, in five different positions and at three different temperatures (8°, 23° and 38°C), with an average daily rate that "must be between -4 and +6 seconds per day". COSC states it tests every watch submitted rather than sampling.
The underlying standard is ISO 3159:2009, Timekeeping instruments — Wrist-chronometers with spring balance oscillator, whose published scope states that it "lays down the definition of the term 'chronometer', describing the categories, the test programme and the acceptable minimum requirements for wrist-chronometers".
What this Codex does not do: quote ISO 3159's definition. The standard is paywalled and only its abstract is public, so the definitional clause itself has not been read. The −4/+6 figure is published as COSC's statement, cited to COSC, and not attributed to ISO.
Sources
Every statement above traces to one of these. Read on the date shown.