See also QAA: Quod Assumpsi, Assumpsi.
Quod Scripsi Scripsi means that when something has become canonical (which basically is the case when someone writes something and no one objects), it cannot be changed or undone. It is a very important principle in IB; without it, things would become a mess. Imagine that you write a book along with someone else. He/she writes a paragraph, then you write a paragraph, and so on. Now what would happen if he or she halfway through the project were to suddenly say: "Actually, I don't like paragraph nr. 2, I want to change it completely."? It would mean that everything that was built on it (or around it) becomes invalid! Something you simply can't afford in a collaborative project.
The rule is there for a reason, and that reason is to enforce some cohesion to the work of more than 25 people over several years, some of whom are no longer active and can not defend their parts of this creation. It is a means of reigning in and channeling everyones' creativity; it protects things that have already been worked out; it helps create continuity between all the threads that make up the tapestry and helps ensure that the tapestry is roughly the same shape at both ends.
The rule ought not be a Law set in stone where infractions are punished by outcasting into the Void; but it has to be more than just a cute philosophy or a guidline we can quote and then cheerily ignore when the fancy strikes.
If you want to tell me that Kemr doesn't exist or that the SNOR sacked all of Western Europe or that the NAL ought to be a Spanish realm - then we have to see many good arguments that override all the history and tradition that's been written up thus far.
The rule is more a matter of facts than our opinions. I can't even _begin_ to calculate how many times I [Padraic] 've said "oo, that sounds wonderful" to someone's neat idea; only to have John respond "but we've already established something else..."
Examples of changes in conjunction to QSS
When Daniel Hicken joined IB he had to explain the reason why Mormons would not be based in Deseret. Through explanation of historical facts *here* that M. Hicken believed would apply *there*, QSS was overridden, but only after Padraic and others agreed that the historical imperative was stronger than what had been written. Even then, as can be seen in the article on Lago Grande, QSS did hold sway and there are ethnic Mormons living in Deseret.
When rodlox joined IB he took then's Afghanopakistan * under his wing, but because this territory was largely an unknown, and was only called Afghanopakistan because that area of the world had not been subdivided, QSS largely did not apply and he had tabula rasa with regard to the internal history. When the effects of Afghanopakistan history affect the world at large, QSS applies.
With discovery of a Jewish kingdom in Yemen a broad ranging discussion came to be, leading to the changing of the maps of the Middle East, Oman and UAE being united to form the Thousand Emirates, and Yemen a loose confederation of warring kingdoms/emirates.