The @ symbol and systemctl and vsftpd

The @ symbol is for special services, sockets, and other units where multiple instances can be run.

For instance, getty@.service is the service that provides text login terminals. When you press Ctrl+Alt+F2, getty@tty2.service is started, creating virtual terminal #2.

Another service that uses this functionality is OpenVPN. You can create a file /etc/openvpn/work.conf, configured to connect to the VPN at your workplace, and then systemctl start openvpn@work.service to connect to it. Similarly, you could create /etc/openvpn/home.conf, then start openvpn@home.service if you had a VPN at home. This prevents you from having to create a .service file for every VPN you connect to.

But don't take my word for it. Try it out! Let's create a simple service that outputs a message to syslog. Create a file /etc/systemd/system/echo@.service with the following contents:

[Unit]
Description=Echo '%I'

[Service]
Type=oneshot
ExecStart=/bin/echo %i
StandardOutput=syslog

Notice the %i? systemd will populate that with whatever follows the @ sign when the service is started. So, try starting echo@foo.service:

systemctl start echo@foo.service

Then, check the journal:

 journalctl -n10

At the bottom, you'll see that systemd ran /bin/echo foo:

Feb 24 12:41:01 localhost echo[8412]: foo

Now, try systemctl start echo@bar.service. This time, systemd will populate %i with bar, so you'll see:

Feb 24 12:42:51 localhost echo[8432]: bar

That's all there is to it! Anything could potentially follow the @ sign, as systemd just replaces %i in the service definition with it. OpenVPN uses it for configuration, other services might use for something else, like a port number.

For more information, see man systemd.unit.