Textual content has a long history of not fitting into a single hierarchy. TEI XML encoding went to great lengths to deal with the overlapping hierarchies of analyzed texts. We’ve always dealt with this question in Bible publishing – is the main hierarchy book / chapter / verse, or is it book / heading / paragraph? Both systems were created centuries after the writing, and both are used in nearly all modern typesettings (and both systems see variation among versions!). Which should dominate? And how do we make an ebook that respects the authorial/editorial intent?
Most digital reading systems can only accommodate a single spine within a single hierarchy. ARIA roles, which are used for structural semantics in the content, are designed only to support the single hierarchy of the one reading order of the text. This is clearly insufficient for texts that have multiple hierarchies. It is also insufficient for all readers, for it assumes that all readers will approach the text with the same purpose, the linear and hierarchical reading of the whole.
Digital reading systems should instead support and express whatever textual hierarchies and reading orders the authors of the text -- or its readers -- wish it to. Multiple reading orders, multiple overlapping hierarchies, all accessibly provided according to the authors or readers wishes.