also, somebody please explain what that means
```
error: Package ‘vanilla-dmz-0.4.4’ in /home/maciej/Projekte/nixpkgsv/pkgs/data/icons/vanilla-dmz/default.nix:30 has an unfree license (‘cc-by-nc-sa-30’), refusing to evaluate.
```
I mean, it's not proprietary, what other restriction is there that makes it an unfree license?