‰ is per mille
‰ or 'per mille' means parts per thousand.
I wasn't aware of it until today when I idly Googled "permil", my imagined variation on "percent", to find that this is one of the many spellings of a real thing. Rarely used in English, but more common in other European languages.
Now that I see the symbol, I remember seeing it as a child, on an old typewriter that my Grandfather used. What's old is new.