reference condition meaning

[shaharhada]

In the link: https://www.fluke.com/en-us/learn/blog/electrical/electrical-glossary
written in this glossary in letter A:
"Accuracy:
The accuracy of a digital tester is defined as the difference between the reading and thre value for a quantity measured in reference conditions..."
What is the meaning of reference condition?
(I saw the definition in below but I don't understand it)

 

[Ralston18]

A known, proven quantity.

For example: A source that is known and absolutely proven to be 9.00 volts. Maybe + or - some error range such as 0.01 volts.

Then you calibrate your digital tester to read 9.00 volts when connected to that reference source/condition.

Key is that that reference source must be accurate and within error tolerances.

You cannot just pick up some 9.00 volt battery and have real confidence that the battery is really 9.00 volts.

Could actually be somewhere between 8.50 to 9.50 volts and calibrating the meter to read 9.00 volts would result in voltage measurments that could be + or - 0.50 volts in error.