If the GameObject that this PanelRenderer component is attached to has a parent GameObject, and that parent GameObject also has a PanelRenderer component attached to it, this value is set to the parent GameObject's PanelRenderer component automatically.
If a PanelRenderer has a parent, you cannot add it directly to a panel (PanelSettings). Unity adds it to
the parent's root visual element instead.
The advantage of placing PanelRenderer GameObjects under other PanelRenderer GameObjects is that you can
have many PanelRenderers all drawing in the same panel (rootVisualElement) and therefore able to batch
together. A typical example is rendering health bars on top of characters, which would be more expensive to
render in their separate panels (and batches) compared to combining them to a single panel, one batch.