The amount of cash which are circulating on the market and they are tradeable by the public. It's akin to thinking about shares available on the market (not held & locked by insiders, governments). Derive general public critical from current private key by using openssl in terminal (take out prefix https://directoryethics.com/listings12706285/the-smart-trick-of-%E9%92%B1%E5%8C%85%E9%9D%93%E5%8F%B7%E5%9C%B0%E5%9D%80%E7%94%9F%E6%88%90-that-nobody-is-discussing