iii. Iran reaffirms that under no circumstances will Iran ever seek, develop or
acquire any nuclear weapons.

Why doesn't it say "agrees?"

af·firm

\ə-ˈfərm\verb

: to say that something is true in a confident way

: to show a strong belief in or dedication to (something, such as an important idea)

law : to decide that the judgment of another court is correct

Full Definition

transitive verb

1

a  :validate, confirm

b  :to state positively affirm

edhis innocence>

2

:to assert (as a judgment or decree) as valid or confirmed

"To say that something is true in a confident way"? What if you cross your fingers behind your back? That definition does not mean the affirmer is being truthful, merely that he is stating confidently that the referent statement is true. That definition does not require truth or even sincerity on the part of the affirmer.

Now, the second definition as a transitive verb, and the example is almost identical to how the word form is used in paragraph iii...No, that doesn't require truth either, only that the statement be made "positively," stronger than but similar to "confidently," and the example given, "he affirmed his innocence" (of which the undersigned is not unfamiliar) is consistent with that definition, it's a pro forma direct statement which is not the same as a true statement.

The way that "affirm" is used in courts of law in America however is the way we in which we would expect to find it in a legal (Is JCPOA a legal document?) document such as JCPOA. Sometimes witnesses, for religious reasons, can not "swear" the oath of truthfulness. They are required to "affirm." "Raise your right hand: Do you swear or affirm to tell the truth, the whole truth and nothing but the truth, so help you God." With "I do" the witness is legally bound and subjects himself to arrest for perjury if it can be proved he lied.