We just made the r3 release of Zorin OS 16 Core fully available a couple of minutes ago (after it synced to all of our mirrors). This incremental release is simply a re-build of the .iso file with the latest software updates pre-installed (just like our r1 release in August). If you previously installed Zorin OS 16 there's no need to download this new .iso file.
Let's Encrypt (the SSL certificate authority we use on our servers, including our package repository servers) recently updated to a new certificate authority file. As a result, the system's SSL certificate authority files (provided by the
ca-certificates package) needed to be updated to the latest version in order to correctly validate the newly issued SSL certificates. Because the previous Zorin OS 16 iso included the older
ca-certificates package, whenever someone installed Zorin OS 16 using the previous .iso file and opened the Software Updater for the first time, they would see a "Certificate verification failed" error message.
The newer release includes the latest
ca-certificates package which solves this problem and provides a much smoother experience for new users.
We briefly uploaded an r2 release with the above fix pre-installed. However, there were some security updates to the kernel as well as new NVIDIA drivers released shortly after we uploaded the r2 iso, so we created the r3 iso to include these updates as well, again to make the user experience smoother for first-time users.
To avoid confusion in the future, we occasionally create revised .iso files (denoted with the "-rX.iso" suffix) which include the latest software updates pre-installed. They're primarily intended to make new installations easier and faster, but don't serve any purpose for existing Zorin OS installations.
You can see the details about these software updates by opening the "Details of Updates" in the Software Updater.
r4 is currently mirroring and includes a fix for a theming regression in the Ubiquity installer which made the "Install alongside" option hard to see. We needed to re-build the .iso files as this issue affected the installation process.