I am doing an optics simulation involving transmission and reflection coefficients very close to 1, such as 0.999. While I was an undergraduate student, a professor mentioned that, in certain fields, people characterise such numbers by the number of '9's after the dot. I tried to find any mention of this on the internet, and a name for that number of '9's, but didn't find anything.

Could anyone point me in the right direction?

Incidentally,

1. The formula for that number of '9's in the number x is -log_10(1-x).
2. This might well be an engineering question.
3. ChatGPT suggests that that number of '9's is called 'nines'. Makes some sense, but the rest of the internet seems to be agnostic to this term.