Buying a call means buying a call option, becoming a long position, and paying the corresponding option fee C. When the future spot price St is higher than the agreed price X, exercise the right, buy assets at the price X, and earn ST-X-C. When the future spot price ST is lower than the agreed price X, exercise the right and lose an option fee C.

Selling a call option is just the opposite of buying a call option, that is, selling a call option becomes a short position. When the future spot price St is higher than the agreed price X, the bulls exercise their power to buy assets at the price X, and the bears lose c-(St-x). When the spot price of futures St is lower than the agreed price X, the bulls do not exercise their power and the bears get the option fee C.