Question regarding "Digits"

Hi.

I'm new here, and I looked through some of the thread topics outside Market. I couldn't find a description anywhere of what is meant by "digits" when people say "Buying/Selling X digit Steam Account." What is that referring to? I'm sorry if I'm posting this in the wrong place, but again, I didn't see an announcement telling me where to post these kinds of answers. Anyways, responses are appreciated.

I'll probably be selling my account soon, so keep an eye out for it

 

Digit's show how old the account is. To find the digit type status in console in game.