What is system.stateVersion in Nix?

[phucleeuwu]

I'm trying to understand the purpose and implications of system.stateVersion in NixOS and Nix Darwin.

Questions:

1. What exactly does system.stateVersion control?
2. How does it affect system upgrades?
3. What happens if I change it to a newer version?
4. Is it safe to set it to the latest NixOS/Nix Darwin version when installing a fresh system?
5. Does it impact package versions or just system-level configurations?

From my understanding, it determines compatibility with older versions of NixOS/Nix Darwin, but I’d like a deeper explanation of when and why it should or shouldn’t be modified.

Thanks in advance for any insights!