A very good answer is provided by Yuanky (a meter manufacturer) here - "What does 5(20)A on the meter mean?"
I would not normally quote such a larghe portion of a web page but in this case it needs to either be all, or massive paraphrasing, to make sense.
So, they say (lightly edited):
What does 5(60)A on the meter mean?
There is a parameter 5 (60) A written in the prominent position of the electric energy meter.
The current in parentheses – 60A in the example, refers to the maximum rated current of the energy meter. Different from other equipment, the rated current of the electric energy meter is greatly affected by environmental factors, so a certain margin is generally left when it leaves the factory – the actual maximum rated current is 120% of the marked current.
Therefore, if the number in parentheses is 60, its maximum rated current is 72A – if it is not a particularly harsh environment, the impact on the maximum rated current will generally not reach 20%. Therefore, the maximum rated current of a meter marked with 60A is generally about 66A in actual use.
When this value is exceeded inaccurate measurements occur – maybe more, maybe less.
Current outside brackets
The 5 outside the parentheses here is called the basic current, also called the calibration current. It is determined by the starting current of the electric energy meter – the minimum current value that allows the electric meter to continuously rotate and continuously measure.
The starting current of a general smart meter is 0.4% of the rated current. That is, a meter with a rated current of 5A will be charged as long as the current in the circuit reaches = 5A x 0.4% = 0.02A = 20 mA when in use.
This section is unclear (to me):
There will be a ratio between the rated current and the maximum rated current, such as 5 (60) A, which is a 4 times relationship. This ratio is called “load width”. Generally, there are 2 times, 4 times, 6 times, 8 times or even more than ten times – the larger the load width, the stronger the technical level required, and the price of the meter will naturally be higher.
Therefore, the numbers outside the parentheses have little to do with the actual use by the user—more or less than this value will not affect the metering of the meter. There are mainly two aspects affected by the calibration current: the price of the meter (related to the load width) and the starting current (calculated by the calibration current).