Wikipedia:But for Napoleon, it was Tuesday

Edward Jenner was an English physician and scientist who was the pioneer of smallpox vaccine, the world's first vaccine. He also lived during the Napoleonic Wars, and asked Napoleon to release some British prisoners of war. It is reported that Napoleon said "Ah, Jenner, je ne puis rien refuser a Jenner" (Ah, Jenner, I can refuse him nothing). For Jenner, it was an important political achievement: it proves that his recognition was so high that even a world leader waging war against his country respected him. But for Napoleon, it was Tuesday.

What does this mean?
Unlike other encyclopedias, Wikipedia has specific and standalone articles over several things: events, places, biographies, institutions, etc. There is a side consequence of it: the scope of information that should be included in one article will surely overlap with the information included in another article. It is important to build the web and link the related articles, but there will always be some degree of overlap. Take, for instance, a war: we will likely have articles about the war, battles and campaigns, generals, heads of state, diplomats, the history of the countries that fought in it, etc. Now, let's take a good history book about the war, and improve those articles! Let's see, there's some interesting piece of information here in the book... but in which article should we mention it? The war, the battle, the biography? All of them? None of them? Two men have interacted in some way, should we mention this event in both biographies or only one of them? Should we mention it in the context of the main context, or not?

That's the big question. Even when the topic of the article is notable, that doesn't mean we have to mention in it all the information ever written about it, because Wikipedia is not an indiscriminate collection of information. Some information is significant for X topic, and should be included in its article. Sometimes it is important, but it is not included because there's so much information about X that it was taken to another article that is more specific. Sometimes the information is significant for an article, but not for another, even if it's involved. And sometimes some information has no place anywhere in the encyclopedia, even if we can report reliable sources that mention it.

Yes for X, no for Y
Given two X and Y topics with their own articles, some specific information that involves both X and Y may need to be included in the article about X, but not the one about Y. There are many possible reasons for this.
 * Perhaps it is some routinary action, and similar actions take place on a regular basis. For example: an average Joe saw a concert of the Rolling Stones, and liked it so much that he started his own musical career (which makes him noteworthy enough to meet the requirements of notability). That info may be interesting in the biography of Joe, but not in the one of the Rolling Stones, who play hundreds of times for thousands of people. That concert may had been the most important event in Joe's life, but for Mick Jagger it was Tuesday.
 * Perhaps it is a minor or trivial action, one that passed completely unnoticed for everybody except those directly involved. See the example in the lead: the intervention of Jenner helped to free some prisoners of war, but did not stop the war, nor did the release became controversial or influence the conflict itself. For Napoleon, it was a mere inconsequential administrative action.

Not in the main article
The scope of information that a given article may include does not have the same size for all articles. Barack Obama, for example, will have a biography much longer than that of a pop artist with a couple of singles. In those cases, it is usually preferred to create related articles and move the excessive information to those ones, keeping just a summary at the main one. Still, someone who has just arrived to the article may notice: this thing is mentioned in the news, and it isn't in the article, let's include it! It may be removed for this reason, to keep the article at a reasonable reading size.