This is the cost that cannot be affected by the later decision.
The problem with the cost is that one cannot change the later decision. You have to think back and decide what you would have done differently in the past. You can see that in a lot of decision-making situations where you have a choice to make. For example, if you are deciding whether to buy a house, the cost of the house is the one thing you cannot change. You have to think back and figure out what you would have done differently if you hadn’t bought the house.