This is the notorious "sub-documents" concept.

Change 179940 had a related patch set uploaded (by Jforrester):
[VERY VERY WIP] DocumentSet and subdocuments

https://gerrit.wikimedia.org/r/179940

Notes on transaction history:

https://etherpad.wikimedia.org/p/SubDocumentsVE

Here is a proposal for refinement:

• Conceptually, an article is a set of documents, of which one is the "main document" and the others are "sub-documents". In code terms there is barely any difference between the main document and sub-documents. Documents do not contain other documents, but can reference them, and the references can form cycles. (Essentially the approach from https://gerrit.wikimedia.org/r/269449/ )

• Each document has a unique immutable ID (probably a random string) that never gets re-used for another document.

• In the linear model, documents are appear in creation order (so the main document first), wrapped in an outer 'document' tag:

{ type: 'document', attributes: { id: 'main' } },
...,
{ type: '/document' },
{ type: 'document', attributes: { id: 'xXvWksy' } },
...,
{ type: '/document' },
...

• In the linear model, references into another document contain that document's ID:

{ type: 'mwReference', attributes: { ..., docId: 'xXvWksy', ... } },
{ type: '/mwReference' }

• In memory, each document can be represented as a separate ve.dm.Document and node tree.

• The ve.ce.Document tree for the main document is rendered into a DOM tree using information from the other documents as necessary. The same goes for rendering any sub document as required (e.g. when editing a reference).

• There is *no* special infrastructure in ve.dm.Transaction to handle these extensions, and the document history remains a single transaction list. Rebasing valid transactions over valid transactions will preserve validity (which is what the unique immutable ID was for).