When they say "spar(hard)" it can mean either of two things. Balsa comes in many weights and sometimes the designer just means to use the harder of the balsa spars. It could also mean use a different type of wood such as pine, bass wood, etc. but it usually means to just use the harder balsa as it is the stronger of the different weights (also usually much heavier per unit volume than the lighter grades of balsa. Just a bit of knowledge most do not realize is that balsa is defined as a hardwood as it is a deciduous type of plant. It can be so soft that you can poke your finger through a 1/4 inch sheet or so hard you need a saw or very sharp blade to cut it. Strange also is that a single balsa log can have a very soft part and a very hard section on the same limb.