What is the significance of double @ in the output of `dnf history info`?

Having seen other commands only output single @ for repo names, I am wondering why dnf history info outputs doubles:

Packages Altered:
    Removed abattis-cantarell-fonts-0.301-7.fc36.noarch            @@System

For updates, it outputs single @:

Packages Altered:
    Install snap-confine-2.56.2-4.fc36.x86_64  @updates

But at the same time dnf info outputs single @ for System:

Repository   : @System

When observing carefully you will realize, that System is the only Repository with an @.
Checking with dnf repolist you see non of them having an @.

My conclusion would be: @System refers to the base where is mandatory (like a kernel) and each system has, while the repo is like a tag and can vary by each system.

dnf history info output Packages Altered: at Repository/Repo

If you need an official explanation you can make a question on
GitHub - rpm-software-management/dnf: Package manager based on libdnf and libsolv. Replaces YUM.
and refer to this topic.