Old youtube tutorial: VFB Regions and History
The built-in history allows saving of multiple snapshots of the rendered scene (including region renders) and performing A/B comparisons directly in the Corona VFB. This allows quick comparing of different materials, lighting, or render settings.
To save a snapshot, click on "Store Current VFB" button.
To mark a snapshot as "A" and have it displayed in the VFB, left-click on it.
To mark a snapshot as "B" and have it displayed in the VFB, right-click on it.
To unmark snapshots, simply click on it again, or click on a different one to have it replaced.
VFB History can be used to store multiple "snapshots" of the scene, which can be compared.