We always can make own research.
We can see the occurrence (about 1,190,000 results here; search term, "six figure salary meaning")
For prevalence, we can click result pages 10 or 15, to see if the occurrence remains.
About 5,500,000 results remain, page 15, for the search term, "six figure salary"
It is certain the phrase is in wide use, so it should have an established sense.
We view usage in context, turning to linguistically adept resources, as The Washington Post on household subsidies. "Data from the congressional Joint Committee on Taxation shows that the the vast majority subsidy is paid to households with income over six figures."
The New Your Times tells about the price of education."At a time when the price of a degree from elite institutions is well over six figures, fields such as literature and the arts may seem like a luxury item. ".
We can refer to a dictionary. Cambridge is British.
Webster can tell more about American English reality.
We can draw conclusions. Here, we might have a difference between British and American. In British, a "six-figure" is strictly about the digits, how many of those we need to note a number. Cambridge dictionary says "between 100,000 and 999,999". Webster says "totaling 100,000 or more but less than one million".
"Six figure" is not an audit or accountancy term, anyway. Looking to the occurrence and prevalence, we can tell the matter is a bit psychological. Money talk has its own rules. 100 thousand is more as a threshold. You get to the six figures, when you make 100 thousand. Anything above, can be "over six figure". If you have more digits in your numeral and want to go flashy on cash, you can always say "7 figures". :)