A lot of things are considered biomass, humans included. Bioenergy is predominately focused on conversions associated with plants and not animals, but there are important animal sources of biomass. The important thing about plant biomass is that all plants have similar chemistry, so a chemistry that works with one type of plant biomass can at least be considered for another type of plant biomass, etc. It also means that when you look out the window, everything green you see has similar chemistry and this includes wood items we all see/use daily.