The standard grade over 131 g/dm3 is the heaviest and hardest balsa of the set. A higher density means a closer structure, a firmer surface and better resistance to denting.
That is exactly why it goes into spars, formers, wing roots and anywhere something is seated into the timber or the model carries a load through it. It weighs a little more than the lighter classes, but it is markedly stiffer, and on a part that has to stay straight that trade is worth making.
