The DDLm reference dictionary defines the _alias.dictionary_uri attribute, however, it does not seem to be widely used in the CIF_CORE or any other of the IUCr dictionaries. This information would be useful since it would allow to more easily track down the origin of a specific alias and to double check if the definitions in different dictionaries haven't diverted too much. However, the semantics of _alias.dictionary_uri currently seem a bit unclear to me so it would be great to resolve that first (see issue #481).

Also, there were some discussions in PR #479 of adding the URI to deprecated items. I will prepare a separate draft PR and link it to this issue.