The taiji is the One that contains Yin and Yang, or the Three. This Three is, in Taoist terms, the One (Yang) plus the Two (Yin), or the Three that gives life to all beings (Daode jing 42), the One that virtually contains the multiplicity. Thus, the wuji is a limitless void, whereas the taiji is a limit in the sense that it is the beginning and the end of the world, a turning point. The wuji is the mechanism of both movement and quiescence; it is situated before the differentiation between movement and quiescence, metaphorically located in the space-time between the kun 坤, or pure Yin, and fu 復, the return of the Yang. In other terms, while the Taoists state that taiji is metaphysically preceded by wuji, which is the Dao, the Neo-Confucians says that the taiji is the Dao.
So, if you visualize a Yin-Yang symbol. Taiji would be the circle containing both Yin and Yang (even though most people seem to think that’s wuji), while wuji is the emptiness the entire symbol appears in. In terms of reality, Yin is the object, Yang is the subject, Taiji is containment of subject-object and Wuji is the unknowable, imperceptible void which manifests the subject, the object, and their containment.
So all in all, Taiji and Wuji would be High 1-A+.