If you trade options, it's imperative that you understand the basics of exercise and assignment. You don't need to know all the theoretical details, but you must be prepared for it, especially if you're short options where you don't control the exercise feature.
Exercise is the term used when the owner of a call or put (i.e. someone who has a long position in a call or put) uses his right to buy (in the case of a call) or sell (in the case of a put) the stock. Assignment is the term used when someone who is short a call or put is forced to sell (in the case of the call) or buy (in the case of a put) the stock. Remember, for every option trade there is a buyer and a seller, so if you are short an option, there is someone out there who is long that option and who could exercise.
As you know from the basic definition of options, a call gives its owner the right, but not the obligation, to buy the stock at the strike price. But the seller of a call must sell the stock at the strike price whenever the call owner exercises. A put gives its owner the right, but not the obligation, to sell the stock at the strike price. But the seller of a put must buy the stock at the strike price whenever the put owner exercises.
Remember, on expiration, thinkorswim will automatically exercise any equity option that is $0.01 or more in-the-money.
American-style options can be exercised as soon as they are purchased and all the way up to the expiration date. European-style options can be exercised only on the last trading day before the expiration date. All equity options traded in the U.S. are American-style. But some index options are American-style (OEX), while others are European-style (SPX, NASDAQ). It's very important to know precisely when an option expires, and how it is settled.
Basically, settlement is what you get if you exercise an option. If an option is stock-settled, you will get a long position in the underlying stock if you exercise a call, or a short stock position if you exercise a put. U.S. equity options are stock-settled. If an option is cash-settled, you will get cash in the amount of the difference between the strike price of the option and the settlement value of the underlying index. The OEX and SPX index options are cash-settled.
