When you install a modpack from Modrinth, it is paired and locked to only contain the content from that modpack. By unlocking an instance, you can add your own content. By unpairing an instance, you remove the link between your local installation and the modpack as available on Modrinth. Therefore, when you unpair a modpack, you can no longer automatically update or move between different versions.

To unpair and/or unlock an instance, go into the instance options, then scroll down to "Modpack".