The cash flows that have to be discounted in the NPV method are the incremental cash flows, the additional cash flows from the project. Sunk costs have to be excluded because they are incurred whether or not the project is accepted.
The implicit assumption about cash flows associated with the investment project is that they can be estimated without error. However, in the real world, the cash flows associated with investment projects represent forecasts, and not real values.
The NPV decision rule is consistent with the objective of the firm to maximise shareholder wealth because the maximisation of the NPV increases the market value of the stockholder's share in the firm.
The NPV possesses the additivity principle, which means that when there are mutually exclusive projects, the NPV method indicates that the project with the largest positive NPV should be adopted as it generates the largest NPV of the firm's aggregated cash flows.
The rate at which the present values of the cash inflows associated with a project equal the cash investment. It is the rate that makes the NPV equal to zero.