Every once in a while, a new NixOS release may change configuration defaults in a way incompatible with stateful data. For instance, if the default version of PostgreSQL changes, the new version will probably be unable to read your existing databases. To prevent such breakage, you should set the value of this option to the NixOS release with which you want to be compatible. The effect is that NixOS will use defaults corresponding to the specified release (such as using an older version of PostgreSQL). It’s perfectly fine and recommended to leave this value at the release version of the first install of this system. Changing this option will not upgrade your system. In fact it is meant to stay constant exactly when you upgrade your system. You should only bump this option, if you are sure that you can or have migrated all state on your system which is affected by this option.