American Exercise means that an option can be exercised at any time, and European Exercise means that an option can only be exercised on the expiry date.
Institutional investors need to protect their stock positions by selling options and European Exercise is introduced to support long term protection.
美式期权指的是一个期权在任何时候都可以行权，欧式期权指的是一个期权只有在的到期日才可以行权。
引进欧式期权是由于机构投资者需要就所持有的股票仓位卖出期权起到保护作用。