What is the meaning of “Primarch?”

Question

Reference @evilsoup ‘s comment question which is in follow-up to an associated question, “20 or 21 Primarchs (participating in the Great Crusade)?

“Is "Primarch" a title as recognised by imperial society or is it a description of the physical substance of certain individuals?”

It dawned on me, I’m not sure what the answer to that is.

What is the meaning of “Primarch” — Is "Primarch" a title as recognised by imperial society or is it a description of the physical substance of certain individuals?

Things I am not exactly clear on related to this question, which may help focus the answer: Is it a title or position that the Emperor may bestow, or Primarch-led Legion seconds-in-command may be promoted to or claim should their Primarch fall? Or is it a specific feature limited to only “the 20 (21)” “superhumans genetically-engineered by the Emperor?” @DavidW distills this question down more succinctly:

Is “Primarch” a role or a type of being?

Answer 1

The Primarchs are genetically engineered super soldiers, created using the genetic material of the God Emperor, so they could protect the human race if the Emperor was unable to do so.

The God Emperor was an immortal human, originally born around 8000 BC.

Each Primarch embodies 1 of 20 traits of the Emperor (physical, mental, or psychic) as well as super strength and immortality.

Their genetic material is used to create the Space Marines, which are soldiers that are mutated using the genetic material of the Primarchs and as a whole embody the trait the Primarch inherited from the Emperor.