This symbol is referred to as the Triality Symbol or the Yin-Yang-Yuan Symbol and has been used as a Transgender symbol to indicate three sexes. I think it is applicable to both interpretations and I use it both ways. The Yin represents the feminine energy in the Universe and the Yang represents the Masculine energy. Yuan as used in the Triality is basically the Void, the Unseen, or the Nothing-There in which the elements of yin and yang play out their constant drama. A more practical way of putting it is "the environment." When talking about the Tao, the environment is incomprehensible. But in our more mundane human experience of yin-yang, we can call that which surrounds the everyday, perceptible interplay and expression of yin-yang the environment.
Yin-yang-yuan is simply a more accurate, holistic depiction of the relationship between Tao and human experience, properly portrayed as a Triality.
Retrieved Nov. 2 2009, from http://www.ithou.org/node/1901?size=mini