bcftools isec -n operators

I am still very confused by the use of the bcftools isec -n flag.

According to the manual:
samtools.github.io/bcftools/bcftools.html#isec):

 -n, --nfiles [+-=]INT|~BITMAP
    output positions present in this many (=), this many or more (+), this many or fewer (-), or the exact same (~) files

But after a few trials, I have realized that -n+2 and -n=10 with an input of 10 files do not output the same results.

Has anyone any clues about the use of these operator?