Triple A games ... by what benchmark?

Something which I've always quietly wondered about, but not found sufficient info on the web is: -

What are the benchmarks/grading system games get rated as "AAA", "AA", "A" or something else?

In the "real world", companies and even countries get ratings such as AAA for financial reliability etc, so any examples of how the grading system is applied to gmaes development?

Are 3A games just 3A because they have an enormous budget? Or is it more than just how much cash you can throw around? How many rating types is there? They're surely not official? But some sort of agreed benchmark?

Nothing fancy - Just curious...