The document has grown organically over time as features and clarifications have been added. Unfortunately, this makes the draft harder to read and less structured.

This draft attempts to improve the situation by

• placing a definition of "upload resource" and "target resource" at the beginning of the document (closes Move upload resource and target resource to Conventions and Definitions #2874).
• adding a definition of a "file" and using it consistently throughout the document (we avoided using the term before because resumable uploads can also upload from streaming resources and not only files on disk, but the new definition explicitly mentions that a file can also be a streaming resource).
• introducing a section dedicated to the upload resource, which is the main way through which client interact with resumable uploads (the section houses the description for upload offset, length, completeness, as well as the HEAD, PATCH, DELETE requests).
• splitting the requirements for request handling into client-side and server-side requirements to make their sections more structured.

This PR is not yet completed, but any feedback is welcome.