That’s one thing you don’t have to worry about. What
apt usually lists here is the release a package comes from if that release is known. For Ubuntu, for example, the output might be
zsh/xenial,now 5.1.1-1ubuntu2 amd64 [installed]. This requires that the APT repository be set up in a specific way.
A lot of APT repositories use a somewhat simpler setup without real release information. For such repositories
apt simply says
unknown. Univention’s APT repositories are set up in that simpler mode; therefore all of their packages shown
unknown as the release.
This is a technical detail and absolutely nothing to worry about.