The 6th element, Seed, represents the coordination and regulation of the other 5 elements. We see this in how the palm coordinates the 5 fingers and in how the endocrine system regulates the body functions. We also see it in the pericardium that surrounds the heart.
Without regulation, life could not have developed. Is there a place in the history of Chinese medicine that talks about regulation? --- Yes.
"When our Hearts are well regulated, our senses are well regulated too.
When our Hearts are at rest, our sense organs are at rest too."
Nei Ye, Chapter 49, Guan Zi
And so it is that the elements cannot self-regulate through mutual destruction/discipline in the Heart, as 5-element theory would see it. Their unification and interaction must be regulated and in that way the 5 senses are well regulated too. When our Hearts are at rest (regulated homeostasis), our 5 senses are at rest too. Homeostatic regulation gives rest to the 5 elements.
The 6th element, Seed, with its meridians SJ/Pc, is the element for regulation and homeostasis. It is only found within living beings.
The passage above continues...
"The Heart thereby contains a Heart.
(That is to say) within the Heart there is another Heart."
The Pericardium (meaning peri = around and cardium = heart) surrounds the Heart and protects it. The pericardium provides a protected space for the Heart in which it can be regulated.
Just as the palm surrounds the roots of the 5 fingers, thus coordinating them, the pericardium surrounds the Heart structure giving a space for coordination.
The Nan Jing says, "The source of the Heart comes out at Pc7." Pc7 is at the base of the palm, which surrounds the 5 fingers. The source of the Heart is at the base of the protected space shown by the palm.
and then you tie this back into the spherical Heart and the total integration of meridian theory in the spherical meridian flow, and you have unified theory for a living being.
I end with a quote to wrap (surround) it all together...
"The Heart of Heaven and Earth – Where is it stored? Yin and Yang stimulate it to manifest a sphere of light."