These are service templates, designed to be instantiated with an argument (so the service is
template@argument, running the
template@ service with the given argument). A single service definition can thus be used in different circumstances without needing any hard-coded specifics.
Typical instantiated services you’ll see are per-file system services such as the
systemd-fsck@ service, per-device
gettys, the user manager for each user (
user@), etc. They can be set up in the same way as non-templated services, using
systemctl enable etc., but many are dynamically instantiated by other services.