Each time the edition's manuscript changes, the version before stays. "Versions", beside the manuscript and in the editor, lists them all, newest first: each upload (with the file's name), each save in the editor, a translation's manuscript, and any version brought back.
Compare two. Choose a version to compare from and one to compare to. The comparison goes paragraph by paragraph under each chapter's title: a changed paragraph shows the words that went and the words that came, added and removed paragraphs stand out, and paragraphs that didn't change are folded away. When only the styling of a paragraph changed (italics, bold), it says so.
Bring one back. "Bring back" makes an earlier version the edition's manuscript again: the next ebook and print builds use it, and so do translations and store kits. Nothing is lost: the versions after it stay in the list, and you can bring any of them back in turn.
If the editor has unsaved changes when a version is brought back, they stay with the manuscript they started from; the editor says so, and you can discard them.