chief delegate; head of a delegation with full powers
Formal diplomatic term for the senior representative authorized to negotiate and sign on behalf of a government or organization.
He attended the international conference as the chief delegate.
全権 means 'full powers' or 'plenipotentiary' and can be used alone or in compounds; 主席全権 specifically refers to the chief delegate holding such authority.
Compound of 主席 (chief seat, head) and 全権 (full powers, plenipotentiary).