Maybe I didn't research this enough, but I could not find any measurable way to back up calling oneself an international bestselling author. Is it bunk? Many of you know that there is a way to become a US bestselling author: you need to rank in Wallstreet Journal, USA Today, or, of course, The New York Times. But what of international?
Honestly? No confirmation on this, but I know of a group who used to use that tag when they hit the top 100 of their main genre in a non US Amazon store. Almost all of them were KU authors, so take that for what you will.
I am sure there is a measure for International, as I see authors still using it, but as near as I can tell it appears to be more of a subjective measure based on some random ranking system because no one ever says exactly where they are international bestseller.
I just got 3 #1's in the UK, 1 in the US, and 1 in AUS. I guess that makes me an international bestseller.

Top 100 in anything is not a bestseller on Amazon, and no-one should be claiming it is. That would make me an international best seller for 7 years running, which is ridiculous.
The Trad bestseller lists are irrelevant to Indies.
But on Amazon, imo you need to get #1 in the larger categories before you can really claim anything. International means you need to do it in the same category in at least 3 of the main stores at the same time with the same book. Which counts me out this time, as I only got Arthurian in 2, and I doubt that's really a big category.
I'd say I've done it twice, back in 2018, where I scooped up Space Opera twice, which to get means getting about ten sub-categories around it first. Alas, back then I wasn't looking at the UK and Aus charts, so I can't be sure.