A model is a stack of layers, and a render is those layers painted in order. What each layer shows depends on the parameters it listens to.
Order
Every layer carries a priority, and the stack is painted from the lowest upward. That is the whole rule: to put the frame in front of the fabric, give it a higher priority.
Off: if nothing matches, the layer's first image is used, so something is always painted. On: the layer is skipped entirely.
A parameter value can flip this layer horizontally, so one left armrest covers the right one too.
When a layer has no image for the current combination
This is the case worth understanding, because it decides whether a gap becomes a visible hole.
A normal layer falls back. If no image matches the selected parameters, the layer’s first image is used, so something is always painted.
An optional layer is skipped. No match means the layer simply is not drawn. Use this for parts that genuinely may be absent, an armrest, a headboard, a cushion.
So “optional” is not about whether the layer matters. It is about whether nothing is a valid answer for it.
Mirrored parts
A layer can be marked as allowing a pivot, and a parameter value can then flip it horizontally or vertically when it is composited. One image of a left armrest covers the right one too, which halves what has to be rendered and keeps the two sides identical by construction.
What to reuse
Troubleshooting
A layer shows the wrong image
It matched a different combination than you expect. Check which parameters the layer is tied to: a layer only sees the ones related to it.
Something disappears in one combination
That layer is optional and had no image for the combination. Either add the image or make the layer non-optional so it falls back instead.
