Use Appearance
When Use Appearance is selected all parts in the model are sorted by appearance instead of the default physical material type. Also the board type list title will indicate (Appearance) on the lumber tab.
When maps are created, parts are placed on the corresponding board types (appearance/thickness). Typically one would apply the expected appearance to the various parts in the model giving it a realistic look. You can also create your own named appearances and applied them to a sub-group of parts. When the map is created the sub-groups will be arranged together on separate boards.