Commons:Category inclusion criteria
Commons, like Wikipedia, shouldn't have an indiscriminate number of categories. Most of those listed at Wikipedia:Overcategorization apply here apart from SHAREDNAME since topic categories function in a similar way to Wikipedia articles. For this reason on a disambiguation page a category for the surname will be included such as Category:Lincoln includes Category:Lincoln (surname) where people with that surname can be found.
Most of the time, if you think a topic category shouldn't exist this is because the topic isn't notable. If this is the case then you should probably first nominate the files in the category for deletion; then if the category is emptied as a result it can simply be deleted as empty.
Since many "set" categories shouldn't directly contain any (or many) images directly it is generally not helpful, for example (per SMALLCAT), to have a category for "X in Y" when there are only a few Xs in Y. For example, there are only 2 cities in Devon and since any images should be added directly to those categories (unless possibly files discussing both as cities but that's unlikely and those could still probably simply be added to both individual categories instead) a Category:Cities in Devon wouldn't be helpful. That said, topic categories can and often do only currently contain a few images such as Category:Brosdale Island, currently (as of May 2020) containing only 2 images but anyone could upload dozens of images about the island and all could be added to the category and suddenly it would have hundreds of images. Similarly, a category for "Churches in X" is probably not a good idea of X only has 1 (or a few churches). Since churches usually are taken as notable, any images for them should be added to the topic categories for them. However a category for "Buildings in X" probably is a good idea since X probably has hundreds of buildings and some might not be considered notable enough to have separate categories here.
Generally, a category is appropriate for Commons if any of the following hold:
- The category contains a reasonably large number of images or other pages (use 10 as a rule of thumb, but there is not yet a consensus on this matter).
- The category has a subcategory.
- The category has a Wikipedia article and/or clearly meets Wikipedia's notability criteria.
- The category is part of a diffusion scheme, and at least one of its sibling categories meets any of the other criteria above.
These apply to topic categories, the Wikipedia guideline mainly focuses on set categories. Sometimes these might be excluded by reasons on the Wikipedia guideline such as arbitrary inclusion criteria even though they have a Wikipedia article, for example Category:North Britain was deleted at Commons:Categories for discussion/2018/06/Category:North Britain even though w:North Britain still has an article because "North Britain" doesn't have a definite meaning and thus different parts of Britain could be added to it depending on the editor's view on what is northern. However as noted the category could be re created if we have media specifically discussing the idea of what "North Britain" is.
Unlike Wikipedia, which attempts to determine prima facie notability based on coverage in external sources, the decision to subcategorize on Commons is often driven by the needs of Commons itself. For widely photographed locations such as national parks, it may make sense to have a category for each attraction even though they might not be notable enough for individual Wikipedia articles. If a parent category is a very widely photographed object, such as the Eiffel Tower, it may make sense to subcategorize by decade or even by year. But please do not do this for every subject, only those that need diffusion; use editorial discretion.
An example of a diffusion scheme is Category:Water reflections of clouds in the United States by state. Clearly, Category:Water reflections of clouds in New York (state) has enough images to warrant a category, so Category:Water reflections of clouds in Iowa is appropriate despite having only one image in it.