It’s less well known, but there are a lot more criteria than the simple "-4 to +6s /day" that are needed to be satisfied to qualify as a chronometer. ISO 3159 1976 sets it out to be precise.
Basically, over 15 days, the watch is tested in various positions at 23 deg C and the daily variations measured. Three are vertical: (6H) crown left, (3H) crown up, (9H) crown down. Two are horizontal: (FH) dial down and (CH) dial up. Interestingly, the watch is never tested in position 12H crown right.
Low and high temperatures (8 C and 38C) on days 11 and 13 also feature. Based on these measurements, 7 criteria are calculated, which must all be met and for movements of a diameter over 20 mm, as follows
1. Average daily rate over the first 10 days at 23 deg C ‘M’: -4s +6s
2. Mean variation in rates ‘V’ is the arithmetic mean of the five absolute values of variations in rates obtained for the five positions of the watch during the first 10 days of the tests. Note: The variation in rate is the difference between two consecutive daily rates in identical environmental conditions.: 2s
3. Greatest variation in rates ‘Vmax’ is the absolute value of the greatest of the five variations in rates with regard to the five positions of the watch during the first 10 days of the tests.: 5s
4. Difference between rates in H & V positions ‘D’ (actually 6H and CH and based on averages): -6s +8s
5. Greatest deviation in rates ‘P’ is the absolute value of the greatest of the ten differences between one of the first ten rates and the mean daily test rate.: 10s
6. Thermal variation ‘C’ is obtained by subtracting the rate at 8 deg C from the rate at 38 deg C, the whole being divided by the temperature difference, expressed in degrees Celsius. : +/-0.6s
7. Resumed rate ‘R’ is obtained by subtracting the average of the first two rates from the last rate.