Each version of a document is a set of signed change blobs. Each change names the earlier changes it builds on, so the changes form a graph with no cycles, a DAG. Every change is signed, so you can see exactly who made each step. To show a document at a version, the changes are sorted deterministically and applied in order. Concurrent edits merge by fixed rules, and the last save does not simply win. Documents explains the ordering and the CRDT.

A Ref is the small signed blob that says "at this path, the current version is these change heads." Refs make a document visible at an address. Only the space owner or a holder of a capability can publish a Ref that readers will honor.

Several authors can work on one document. The owner grants a capability to each collaborator. Collaborators publish changes and Refs, and readers accept those Refs because the capability checks out. If someone without permission publishes a Ref for a document, readers ignore it and the document does not move. See Permissions.

The team calls this part open editing. You can change a document you cannot write to, as long as you publish your version in your own space. You publish a Ref at a path you control that points at the original document's changes plus your own. That creates a new document URL that you own. The full history stays intact, and every earlier change is still signed by its original author.

If the original author likes your work, they can update their own Ref to include your changes, without granting you any future rights. So Refs give you branches and change requests, much like pull requests, in the protocol itself. The model comes from git, the most successful distributed version control system. One difference: in Hypermedia, identity and permission live in the data, where git leaves them to the hosting service.

Comments use a lighter mechanism for edits. A comment blob carries its whole content. An edit publishes a new blob that replaces the old one and keeps the same identifier. Small, single-author pieces stay cheap and still leave a version trail. Documents are larger and have many authors, so they get the full change graph.

Merging two concurrent Refs back into one, rebase, and revert do not exist as operations yet. The team is also redesigning how a document's location relates to its authority, which affects how moves and branches will work. See Where this is going.