Semantic versioning. 4.0 -> 5.0 indicates that the new society may be backwards-incompatible, whereas 5.0->5.1 implies that society will keep all of its existing APIs while adding more. A patch version that changed its internals without adding or removing any new features would be called 5.0.1, but society versioning doesn't include the last digit because civil servants don't ever get credit for their behind-the-scenes improvements.