First results of the investigation, regarding WikibaseManifest information on Federated Properties enabled Wikibases:

• the Manifest should include some notion of the repo's entity sources in order to be useful for both Wikimedia Commons (mediainfo) and third-party Wikibases with Federated Properties
  • on Wikimedia commons, the entity sources for Items, Properties and Lexemes would be wikidata.org
  • on third-party Wikibases with Federated Properties enabled, the entity source for properties would be the federation source repo
  • an example for the configuration of entity sources can be found in the Federated Properties documentation
  • RDF prefixes could also be included in WikibaseManifest, as described in that Federated Properties document
  • a section such as entities in the prototype examples may be a good place to include entity source information
• other than that, the WikibaseManifest should not include any info that is specific to the Federated Properties feature, such as fedearatedPropertiesEnabled: true/false or federatedPropertiesSourceScriptUrl

Additionally, a third-party Wikibase with Federated Properties enabled could not only be a provider, but also the consumer of WikibaseManifest, when reading it from its federation source repo. This way, settings such as rootScriptUrl and wiki namespace IDs could be taken from the Manifest instead of having to configure them manually in LocalSettings.php. However, at the moment Federated Properties repos are using their local namespace id for properties, instead of the federation source's property namespace id. Also, as long as WikibaseManifest is an extension and not reliably part of Wikibase core, it is of limited use for Federated Properties enabled third-party Wikibases.

Change 624011 had a related patch set uploaded (by Tonina Zhelyazkova; owner: Tonina Zhelyazkova):
[mediawiki/extensions/WikibaseManifest@master] Add ADR 3 - Manifest Output Format

https://gerrit.wikimedia.org/r/624011

Change 624011 merged by jenkins-bot:
[mediawiki/extensions/WikibaseManifest@master] Add docs - Manifest Output Format

https://gerrit.wikimedia.org/r/624011

gerritbot missed to remove the "Patch-For-Review" tag after https://gerrit.wikimedia.org/r/624011 got merged