Options are either American or European. In India, till 2011 all index options were European while all stock options were American. A European option is exercisable only on the date of expiration while an American option is exercisable on any date prior to the expiry. When an option buyer chooses to exercise the option, he goes to the exchange and opts for exercise. Obviously, when someone exercises the option, then some seller will have to take the liability on the other side. This risk is randomly assigned to option sellers. Options assignment used to be a big risk when you sell options. However, since all stock options have become European options since 2011, this is no longer a risk for options sellers. Assignment is a risk that option sellers have to be conscious of in case American options get re-introduced at a later date.