Hello Muhammad,
Thank you for writing in.
This hole diameter is the diameter of the transformer.
Hope this can help you.
Best regards

Hello Muhammad ,
The two potentiometer in the transformer can be used for voltage/current calibration.
Here is the instruction of voltage/current calibration:

Current calibration:
1. In the condition without current, use slotted screwdriver to adjust the “Zero” potentiometer behind the current transformer until the decimal point of the current meter flashes quickly which is the best point. The decimal point is always bright or not bright, which means zero offset to positive. The decimal point flashing slowly means zero offset to negative.
2. Then load the half of the actual measured current to the meter, adjust the “Span” potentiometer beside the current transformer until the displayed current value is the same as the standard meter.

Voltage calibration:
In the condition of half of actual measured voltage, use screwdriver to adjust the voltage calibration potentiometer inside of the meter until the displayed voltage value is the same as the standard meter.

Hope this can help you.
Best regards

Hello Muhammad Asif,
Sorry for causing you misunderstanding.
The voltage calibration do not need to use the potentiometers in the transformer.
For the current calibration, it needs to use the two potentiometers.
Best regards

Hello Muhammad Asif,
Here is the datasheet of the product:
DATASHEET_100384
Hope this can help you.
Best regards

Hello Muhammad Asif,
It is the product of DROK.
The module number just shows the one of the product series of DROK.
Hope this can help you.
Best regards