He was elected chairman of the party.
The President of China visited Japan.
top seat; first desk (in orchestra)
Refers to the principal seat in an orchestra section (e.g., first violin) or the highest-ranking position in a seating order. Can be used with either kanji form, but 主席 is more common for this sense.
She is the first violinist (concertmaster) of the orchestra.
top student; head of the class
Only when written 首席
Specifically refers to the highest-ranking student in a class or school. Written as 首席.
He graduated at the top of his class from university.
He serves as the chairman of the party.
The new leader took office as chairman.
chief judge
chief justice of the Court of Final Appeal of Hong Kong