I assume it goes "Fn" then "Fn+", then "Fn/VF", then "VF-", then "VF", etc, but who knows?
For us there is (summing up longer definitions):
Poor: terrible condition
Fair: pretty bad
Good: significant defects, but possibly displayable as is (depending how lenient you are) and certainly restorable
Very Good: some defects, but most collectors would happily display it as it
Fine: Really wonderful, virtually no defects
Mint: Doesn't exist (unless there really is a place where posters are "minted" like coins)
No pluses or minuses, but "in-betweens" of every grade, so "good to very good" is somewhere between the above two definitions
THIS HAS WORKED INCREDIBLY WELL FOR US FOR DECADES, AND FOR OVER ONE MILLION ITEMS!
Now here is the only controversial part:
We use a "sliding scale" depending on year and poster type.
If ALL items were graded on the exact same scale, then 99% of "newer" rolled posters would be "fine" and 99% of 1910s and 1920s posters would be "fair".
So we adjust the grading scale based on how an item compares to others of the same size and from the same decade.
THIS TOO HAS WORKED INCREDIBLY WELL FOR US FOR DECADES, AND FOR OVER ONE MILLION ITEMS!
It is only controversial because some people have a philosophical argument with it!