Recently while reading some legal treatises, I came upon this statement about the "International Seabed Authority". The statement was that the International Seabed Authority was given

competence to act on behalf of mankind.
This is not to say anything negative about the ISA. However its more about this idea of "competence to act on behalf of mankind". Questions:

How does a corporation, state, legal entity, artificial entity acquire "competence to act on behalf of mankind"?
Did you give them competence to act on your behalf?
How did ISA acquire competence to act on behalf of mankind?
Are there other organizations, legal entities or the like that are acting on behalf of mankind?
If not all countries or men on the planet called Earth are parties to treaty or document creating ISA, how can they competence to act on behalf of mankind?