Unassigned Pipe

The unassigned pipe should only be considered as a way to check that your lighter (or you if you rendered the multipass image you are decomposing / recomposing) assigned all lights and materials in the 3D scene the image was rendered from.

As touched on in the 'Quality Check' page, once all assigned AOVs from a materials or lightgroups breakout are taken from the beauty if any lights and / or materials were not assigned, or denoise filters were applied to only specific AOVs at render time in 3D, the resulting image could likely contain negative values and therefore be unsuitable to add the rebuild. In such circumstances the best practice is to re-open the 3D scene and make the amendments needed so the unassigned pipe is found empty when a new rebuild is made in compositing.

That said, real world constraints might not allow for such amendments and the unassigned pipe may still be used if it's checked correctly. The quality check tools will give you the pixel values for your image and if there are no negative values this is a good start point. Otherwise clamping to 0 and / or color correcting to approximate a match to the beauty might suffice but adding the unassigned pipe to a rebuild should always be checked carefully beforehand.

In AOV_rebuild v1.1 the addditive rebuild and subtractive rebuild come with different unassigned rebuild methods, they both produce slightly different results and I think for layout purposes both suit their assigned rebuild mode. I'll keep testing both but the differences I've noticed so far is negligable.