This is the path to or name of a TOML file containing static configuration parameters for Node. It can contain any parameters other than --data-directory or --config-file (it can contain those too, but they'll be ignored), including the "undocumented" ones. If it's a relative path, it's interpreted to be relative to the value of --data-directory. If it's an absolute path, it's interpreted absolutely. It is never required, but defaults to config.toml. (Which is to say, if you put a file named config.toml in the --data-directory, it will be found and used even if you never specify --config-file in the setup).