Rebuild Mode
There are 2 rebuild modes:
Additive: Starts with an empty image and plusses every AOV shuffled out together to rebuild the beauty.
Subtractive: Starts with the beauty and takes each AOV from the complete image plussing it back one by one for all AOVs to restore the beauty (with modifications where needed).
These distinctions can be important depending on the render you are working with. If your render has unassigned materials or lights, or has denoise filters applied to only specific AOVs (only the beauty for example). If you rebuild with the additive method, comparing against the beauty these differences will be visible.
It also matters when using divide / multiply loops. Material and lightgroup blocks need to rebuild back to the beauty before being merged to the larger rebuild through a divide / multiply loop. This is because the loop shows the difference between the rebuild and the beauty it's compared against. When testing the tool I found this can be problematic for emissive materials as they can also be considered lights in a 3D scene / render engine. This is a wider topic that highlights a limitation of the tool and requires a wiki page of it's own. Navigate to the 'Limitations' page to see this topic discussed in detail.
To summarise the rebuild mode options,
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I recommend the additive method for QCing your lighters work to check for unassigned materials, lights, or denoise filters applied to only specific AOVs.
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I recommend the subtractive method if you need to workaround such issues.