What does service_name@configuration_file.conf do when running systemctl commands?

Question

What does the following systemctl command do?

$ sudo systemctl start [email protected]

I am unfamiliar with this particular syntax. What does the @server.service part of this command do? It looks like it might be pointing at some kind of configuration file called server.service?

Reference:

https://www.digitalocean.com/community/tutorials/how-to-set-up-and-configure-an-openvpn-server-on-ubuntu-20-04