The automatic docs pages are:
https://en.wikipedia.org/w/api.php?action=help&modules=query%2Bwbentityusage and https://en.wikipedia.org/w/api.php?action=help&modules=query%2Bwblistentityusage
Only return entity IDs that used this aspect.
Values (separate with | or alternative): C, D, L, O, S, T, X
It's unclear what these letters mean without knowing otherwise, or digging into the code to find out:
* Usage flag indicating that the entity's sitelinks were used as links.
* This would be the case when generating language links or sister links from
* an entity's sitelinks, for display in the sidebar.
* @note: This does NOT cover sitelinks used in wikitext (e.g. via Lua).
* Use OTHER_USAGE for that.
public const SITELINK_USAGE = 'S';
* Usage flag indicating that one of the entity's labels were used.
* This would be the case when showing the label of a referenced entity. Note that
* label usage is typically tracked with a modifier specifying the label's language code.
public const LABEL_USAGE = 'L';
* Usage flag indicating that one of the entity's descriptions were used.
* This would be the case when showing the descriptions of a referenced entity. Note that
* descriptions usage is typically tracked with a modifier specifying the language code.
public const DESCRIPTION_USAGE = 'D';
* Usage flag indicating that the entity's local page name was used,
* i.e. the title of the local (client) page linked to the entity.
* This would be the case when linking a referenced entity to the
* corresponding local wiki page.
* This can be thought of as a special kind of sitelink usage,
* specifically for the sitelink for the local wiki.
public const TITLE_USAGE = 'T';
* Usage flag indicating that certain statements (identified by their property id)
* from the entity were used.
* This currently implies that we also have an OTHER_USAGE or an ALL_USAGE
* for the same entity (STATEMENT_USAGE is never used alone).
public const STATEMENT_USAGE = 'C';
* Usage flag indicating that any and all aspects of the entity
* were (or may have been) used.
public const ALL_USAGE = 'X';
* Usage flag indicating that some aspect of the entity was changed
* which is not covered by any other usage flag (except "all"). That is,
* the specific usage flags together with the "other" flag are equivalent
* to the "all" flag ( S + T + L + O = X or rather O = X - S - T - L ).
* Due to migration efforts, this is currently used redundantly with
* STATEMENT_USAGE or DESCRIPTION_USAGE, as they were only added later.
public const OTHER_USAGE = 'O';
Docs of these aspects also exist at https://doc.wikimedia.org/Wikibase/master/php/md_docs_topics_usagetracking.html
Some sort of better documentation inline on the API output would be useful
* [x] API docs are improved to explain what each available aspect means
* [ ] documentation has been double-checked and improved if necessary to ensure it is up-to-date
* in particular, most of the documentation for the "Other" aspect seems out of date as it seems to actually include Alias-usage and existence checks, but not Description usage
* [ ] cross-checked this documentation with https://grafana-next.wikimedia.org/d/000000160/wikidata-entity-usage?orgId=1&refresh=5m
* [ ] Backported to appropriate/supported release branches of Wikibase