I think they just mean base 12. So “10” isn’t ten, it’s 1 * 121 + 0 * 120; xyz is x * 122 + y * 121 + z * 120.
Like sixteen in hex is 10 (commonly written 0x10, to differentiate it from decimal 10)
Edit: oof, my client is trying to be clever with the mathematical writing and bungling it, I’ll try to fix… Hmm, hope that makes it better not worse