If reinstallation can still practically happen in rolling release, when does that happen?

Does rolling release differ from non-rolling release in that there is only incremental change to the system and packages, theoretically no need for reinstallation? If reinstallation can still practically happen in rolling release, when does that happen? Similarly, does rolling release require less reboots when updating/upgrading the kernel and packages than non-rolling release? When does rolling release require reboot after updating/upgrading? (what is difference between updating and upgrading?) If I am correct, NixOS follows rolling release pattern.