If we need to give an example for the applications of entropy, there is a net heat and mass exchange in engineering systems. For example, in the transfer between the hot coffee mug and the cold environment, the amount of heat energy is transferred from mug to environment. If we calculate the entropy changes of the mug and environment, you will see that the change of entropy in the environment is bigger than in the mug. So, the total entropy generation is bigger than zero. This gives a real idea about how many irreversibilities in these systems in general.