 This can be seen by considering some examples.The aim of chess is to checkmate, but although it is expected that players will "try" to checkmate each other, it is not a rule of chess that a player "must" checkmate the other player whenever he can (as a matter of fact, unskilled players often fail to take the opportunity to do so).Similarly, it is not a rule of football that a player must score a goal if he shoots a penalty (it is only expected, and not required, that he will try).