ZeroTier One allows systems to join and participate in ZeroTier
virtual networks.

ZeroTier One is a client application that enables devices to join
ZeroTier virtual networks (typically configured and managed by a
network controller). It provides an encrypted and secure network
connectivity solution that can be used for a variety of purposes,
including VPN alternatives, remote access, distributed
applications, and more.

Since zerotier-one needs the tun module to be loaded in order to
work, that is done in the start function of rc.zerotier-one.
If you wish you can make the module load from rc.modules.local.

The following can be used to start/stop NordVPN automatically:
/etc/rc.d/rc.local

  if [ -x /etc/rc.d/rc.zerotier-one ]; then
    /etc/rc.d/rc.zerotier-one start
  fi

/etc/rc.d/rc.local_shutdown
  if [ -x /etc/rc.d/rc.zerotier-one ]; then
    /etc/rc.d/rc.zerotier-one stop
  fi
