This describes changes to a system over time. Saving multiple versions in total would capture variability with no distinction of the commonality. Capturing a differences file or delta would distinguish them.
The group had considerable discussion about how to arrange this feature with respect to inter- and intra-system commonality and variability. Our provisional approach unified inter-system and termporal variability, on the assumption that the same representation could be used by convention to express either differences among systems or changes to a single system.