The Versions tab of an engine offers insight into the evolution of the engine over time, as well as control over what happens to the engine next.
You can navigate through the history of the engine on the left side:
The active version of an engine is the one that can be deployed and used for translating text or various XLIFF files.
The next version shows the state of the engine after the last edit, or in other words, what the engine will look like after it gets trained or re-trained. Newly created engines that not have been trained yet only have a Next version.
Depending on the system settings, you may or may not see an inactive engine version as well. You can read more about active vs inactive engine versions here.
The rest of the versions round out the history, showing the composition of corpora over time as well as the versions of stock corpora or engines used.
The next version of the engine is the only one that can be edited, cloned or trained. It displays the current composition of the engine.
To see how the engine has changed since the active version was trained, turn on the Compare to active version toggle. You can see what was added, removed, or updated. The Revert to active version button offers an easy way to discard all changes that have been made since the active version was trained. (Note that updates to stock corpora or stock engines cannot be rolled back.)