The thing is, you cant break the bits up like this. Its non sensical. You have four bits(16 values) if you split the bits up like you said then you have 2 bits or four values to store the variant(oak, birch, etc) and then the other 2 bits for the axis where you only use 3 values out of four. Of which you couldn't use as another variant because then you would lose your axis. So instead it is better to express it in numerical values. Whether it's in base 10, 16, or 2.