https://player.vimeo.com/video/689757958?h=d8ba20bde6&badge=0&autopause=0&player_id=0&app_id=58479
Dragging a material into the Materials list will automatically apply it to the trunk of the tree.
By then highlighting this material, other options will open up below.
Here is a set of options, which is set to the Trunk layer by default.
The other options are Branch and Leaf.
Branch material highlighted in the Materials link field, being applied to the Branch layer in the Assign To parameter.
Clicking on the Hierarchy icon in the link field will assign the material to any child of the Trunk layer that is set.
On the left tree, Hierarchy is disabled, so the material is applied to the Trunk layer only, whilst on the right the Hierarchy is enabled, applying it to Branch layers also.