The Solar Plexus sits where a classical chakra sits, and is one of two positions where the Human Design assignment and the classical one are close to the reverse of each other.
The two schemes
In the classical chakra scheme as it circulates in modern Western usage, the centre at the solar plexus is associated with will, personal power and self-assertion, and the centre at the heart with feeling and compassion.
In the Human Design bodygraph the emotional centre is the one at the solar plexus, and the centre at the heart is concerned with will and drive.
This is recorded as an observation a reader can check against each system's own published description of its centres, not as a claim anyone has made about the relationship between them.
Why it is worth saying
The bodygraph is often introduced as the chakra scheme with two centres added, which is roughly true positionally and misleading here. Anyone reasoning from the classical associations to what the bodygraph means at this position will get it wrong.
The arithmetic
Nine centres against a classical seven, with the G or Identity centre and the Spleen having no classical counterpart at all, and the remaining seven corresponding by position with varying closeness of function.
Human Design was published by Ra Uru Hu in 1987 and is recorded here as a contemporary system.
Sources Rainbow Body: A History of the Western Chakra System from Blavatsky to BrennanKurt Leland with The Human Design SystemRa Uru Hu (Alan Robert Krakower)