Who is allowed to sign
A signature proves control of a key. It does not prove who used it, because a person at a terminal and unattended software holding the same key produce identical bytes. So the document says, separately for forming the deal and for amending it, what the other side is entitled to expect behind the signature.
The three values
A valid signature is enough, which is what a signature on its own already meant. The owner is bound by whatever holds the key.
The runtime holds the approval pending until the account's human confirms it, with a credential that attests they were present, and records that confirmation beside the signature.
Both are on the record: the agent's signature first, and the person's confirmation second. It is for parties who want both.
In the signed document this is one line inside the change control clause, such as "approval": { "formation": "agent", "amendment": "person" }.
The defaults
When the document says nothing, forming defaults to agent and amending defaults to person. A published offer at a published price is a bounded commitment, while a change to a live engagement is where an agent that approves automatically is most likely to be attacked.
The grade follows the deployment. Where formation completes somewhere that can hold it pending, person is enforced. Where it cannot, the confirmation is only a record and the grade is evidence.
What the confirmation proves is bounded too: a verified human was present and confirmed. It does not prove they read the document or understood it, and nothing that renders one of these documents may suggest otherwise. The rules are in section 6.1 of the specification.
The organization behind the person
A document may also say which organization a party is engaging for, and which mandate the approving person was acting under. Both are optional fields, both sit inside the signed bytes, and both are read against the Agent Mandate standard.
A person-grade approval on its own proves a verified human was present. With a mandate beside it, the other side can also see that the human held current, unrevoked, traceable authority to commit the organization to this much money for this kind of work. What a court would make of it is outside the system, and nothing here claims otherwise. Documents that carry neither field remain conformant, and this is set out in section 6.2 of the specification.