The new extends attribute for references allows referencing specific parts on a reference as sub-reference ( e.g. a page on a book ). The reference tag that 'extends' it's parent reference will then only contain text to specify it's parent more precise. - When using this text alone while editing or previewing such a sub reference it might be confusing to only see that text ( e.g. page 23-42 ).
Make sure that viewing and editing references in the Android App supports sub references and provides context so they can be understood.
T243839: Make the iOS App compatible with BookReferencing
T242899: Make MobileFrontend and MinervaNeue compatible with BookReferencing