Hi, "Reassign Material" is not for that. This button rebuilds the auto-material that Forest generates
and assigns to itself, when "Consolidate Material" is on.
It contains all materials used in the Geometry List. This option is rarely used, because it should work well by default.
There is not any option to reassign the material from custom object. Anyway, if you "clear" it, Forest will use the material assigned to the custom object, so this Material parameter is optional.