On Commons Categories should be disambiguated like Wikipedia articles, but there is a greater emphasis on accuracy over conciseness, because on Wikipedia readers can figure out the article's topic based on its content, whereas on Commons categories are often undescribed and users must infer the topic from its parent categories and existing media.
Accordingly, there should be a higher threshold for primary topics. Generally if Wikipedia wants 75% in this specific case, we want 90% the others combined or if the topic is overwhelmingly more important than others with that term. If at least 90% of people who search for "London" want the capital for example the category should be called "London" not "London, England" for example (with the dab page at Category:London (disambiguation). However when this isn't the case as with the categories for Mercury or Perth then they should be disambiguated because they fall below this. Categories however should not be preemptively disambiguated unless a specific guideline states otherwise (like USPLACE) or because of a good reason and shouldn't be disambiguated simply because of part title matches unless there is likely confusion in a general context. For example we reasonably expect to see Abraham Lincoln at Lincoln (disambiguation), but very few sources would refer to the waltz composer Harry J. Lincoln by an unqualified "Lincoln", so he would only be included in the Lincoln (surname) category and not listed at Category:Lincoln as well. There is no single criterion for defining a primary topic. However, there are two major aspects that are commonly discussed in connection with primary topics:
- A topic is primary for a term, with respect to usage if it is the topic sought when a reader searches for that term at least 90% of the time.
- A topic is primary for a term, with respect to long-term significance, if it has substantially greater enduring notability and educational value than all of the other topic associated with that term combined.
Commons has a scope which far exceeds that of Wikipedia. In general, any publicly visible place or object will have people photographing it and uploading photos of it to Commons. Even if the specific thing is not notable, it can be used in a Wikipedia article about a broader geographic region (e.g. a waterfall in a national park) or in a Wikipedia article about the general class (e.g. an article on waterfalls). Whenever you wish to create a category for an entity that sounds potentially ambiguous, search on Google to see if other possibilities exist. For example, "Pier A Park" in Hoboken, New Jersey, sounds like it could apply to any pier labeled "A" and then turned into a park, but an extensive search (e.g. '"pier a park" -hoboken -nyc -york -jersey') turns up no significant results hence Category:Pier A Park is acceptable. On the other hand, disambiguation is wise for Category:White Memorial Fountain (Stanford University) even though its Wikipedia article is unique, as '"white memorial fountain"' turns up many other results such as one in Boston, and in fact there is a Commons category for a fountain in Connecticut. When in doubt, disambiguate.
Keep in mind that a topic may have more than one category associated with it such as the city Cambridge also having the university or Wells covering broadly all types of wells. If there is a primary general meaning that should be at the base name. Additionally, we should keep in mind certain differences between the type of material available on Commons vs. Wikipedia. For example, Commons categories are much more likely to be in plural, and copyrighted works (even very notable ones) are unlikely to have many files associated with them on Commons. That is why the primary topic of Category:Friends is the general noun but the primary topic of Friends on Wikipedia is the sitcom.
Note this briefly explains disambiguation for categories on Commons, but to avoid duplicating much content, please refer to Wikipedia's disambiguation guidelines.