Ti (体) and yong (用) can be understood in three different ways: 1) a physical thing and its functions or roles; 2) the ontological existence of a thing and its expression and application; and 3) the fundamental code of conduct, and its observance. In any ti–yong relationship, ti provides the basis on which yong depends.
Tian (天) means heaven in the physical sense, while qian (乾) means its functions and significance. (Kong Yingda: Correct Meaning of The Book of Changes)
What is most subtle is li (理), while what is most conspicuous is xiang (象). Li as the ontological existence and xiang as its manifestation are of the same origin; there is no difference between them。(Cheng Yi: Cheng Yi’s Commentary on The Book of Changes)