What does heavenly stems and earthly branches stand for?

The eight-character heavenly stems represent the sky, the earthly branches represent the earth, and the hidden stems in the earthly branches are human elements, representing people, and are the three types of life laws between heaven and earth.

The eight characters talk about personal destiny with the sun as Tai Chi, in which the sun represents oneself, the sun represents spouse, and other sun represents people or things corresponding to the external five elements.

For example, the nail wood represents the head, the ground branch represents the interior of the main body, and the abdomen is dirty. For example, fire represents the heart.