An issue that may arise during normal FTP use is that you may want to block 'problem' user who abuse your system or simply only allow a specific organization to access your server. This option will allow you to block any IP or range of IP addresses. You can alternatively block all addresses and only allow certain IP ranges to access your server.
Starting in v2, the option to set this on a per user basis has been added ("security" - "access rules") and should be used going forward.
access rules