This is useful where root access is not available.
MASSIVE WARNING! Do NOT enable this option unless you totally know what you are doing, as naive use of this option can create massive security problems.
Specifically, vsftpd does not / cannot use chroot technology to restrict file access when this option is set (even if launched by root).
A poor substitute could be to use a deny_file setting such as {/*,*..*}, but the reliability of this cannot compare to chroot, and should not be relied on.
If using this option, many restrictions on other options apply.
For example, options requiring privilege such as non-anonymous logins, upload ownership changing, connecting from port 20 and listen ports less than 1024 are not expected to work. Other options may be impacted.
default=NO