What's the Central Legal Argument That Otto Makes to Reneera in Episode 10?

In Westeros kings and in Westeros are an absolute monarchy which means that whatever they say goes okay so the king's declaration as to his particular successor has the force of law. It is binding on his successors unless those successors who are legally chosen by him specifically overturned. So yeah Viserys can pick Reneera and legally Reneera is the the legal heir under Westeros he won however. And I was definitely wondering what that last part like he said okay I won't but of course he did.