The primary purpose of depicts (depicts (P180)) statements on Commons is to identify, in a structured way, the items clearly visible in a media file.
Wikidata has many entities at varying levels of detail and MediaSearch can't reliably infer relationships between them. Therefore, to optimize a media file for discovery, you should add multiple depicts (P180) statements, both general and specific. For example, this image of the fictional dog Lassie should have the statements dog (Q144), Rough Collie (Q38650), and Lassie (Q941640).
It's also very important to understand that Structured data on Commons does not function like the Commons category system, currently does not intend to, and it does not have the same clear hierarchy and division.
For example File:Bonnieclyde f.jpg depicts: Bonnie and Clyde (Q219937) as well as depicts: Bonnie Parker (Q2319886) and depicts: Clyde Barrow (Q3320282).
To form a sitelink from a relevant Wikidata item to Commons, navigate in Wikidata to the relevant item (which may be about either a "normal" entity or a Wikimedia category; use the latter if it exists). In the "Multilingual sites" area near the bottom of the Wikidata item page, indicate commons as the wiki, and explicitly enter the Commons category name, with the "Category:" prefix. Any corresponding Wikipedia articles, etc., can be sitelinked in a similar manner
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