Wikipedia:The onion principle

There are many situations in which one topic includes another, either conceptually or physically or both. Australia includes Sydney. Mathematics includes algebra.

This can even have many levels, like the layers of an onion. The outer layers are sometimes called higher levels, giving a broader picture, and the inner lower levels, giving a more detailed view.

But the important thing is, where the topic of one article includes another but the common name for both topics is the same, this structure can be helpful in deciding how to disambiguate and link to the articles concerned. Our priority here is reader experience. In general, it's far better to send someone who wants to know about the inner layer (the lower level) to an article on the whole onion than it is to send someone who wants to know about the whole onion (the higher level) straight to the middle layer.

Ideally we want everyone to go straight to the article they want of course. But that isn't always possible.

When in doubt, go to the top

Disambiguation
If topics covered by two different articles share one name, then the term is ambiguous and it is necessary to disambiguate. If there is a primary topic then the base name should lead directly to the article on that topic, either by being the title of the article, or by being a redirect to it. If there is no primary topic, then the base name should similarly lead directly to a disambiguation page.

Very occasionally however it is impossible to tell whether there is a primary topic and if so which it is. This occurs for example with some geographical articles on minor towns and regions, which may have had no significant coverage in reliable sources. It is rare, because a topic which has had no such coverage is unlikely to pass the notability tests and have an article at all, but it does occur.

In these few cases, it is very common for one of the topics to include the other, for example a small town often shares the name of its locality. When this occurs, with the locality including the town, it is conventional to give the base name to the locality –  the outer layer of the onion. Similarly if a town includes a village by the same name, the base name is given to the town. (Another option is of course to merge the two articles.)

Linking
When a wikilink in an article points to an ambiguous title, it is sometimes not obvious which sense is intended. Most commonly, this occurs when the text containing the link is unreferenced and the link points to a disambiguation page. Fixing such links is part of copyediting.

There are three possible ways of approaching this:
 * The link can be simply removed, perhaps along with the unreferenced text of which it is part.
 * The link can be left pointing to the disambiguation page, or pointed there if it currently points somewhere else and there is reason to doubt the accuracy of this link.
 * If one of the possible targets includes the other(s), it makes better sense to point the link to the most inclusive of these possibilities –  again, the outer layer of the onion. For example, a person born in New York City was born in New York State, but not all of those born in New York State were born in New York City. By pointing the link to New York State, the article becomes less precise, but it is now at least correct, regardless of which meaning was intended.

None of these fixes is as good as finding a reference for the information and making the link precise, accurate and referenced (and possibly adding other content, once you have this source to hand), but each is appropriate on occasions.