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. Hard balsa is the useful middle ground, lighter than ply and still carvable with a blade, which is why it goes into wing roots and local doublers rather than into whole formers. Where a part has to hold a screw or a hinge, Finnish aircraft plywood is the stronger answer, and the lighter grades for sheeting and infill are in balsa sheet for models.
