2.1.8 Revision
A revision specifies the state of an object space (section 2.1.4), which is comprised of a choice of root objects (section 2.1.7), a set of objects (section 2.1.5) reachable from the root objects via object references, and the state of each object.
A revision is identified by an ExtendedGUID structure (section 2.2.1). A revision is immutable. Revisions that specify the same ExtendedGUID structure identity MUST resolve to an identical set of objects, object states, and root object choices.