Classical NPV assumes the firm commits irrevocably at time zero. In reality, managers can defer investment to learn more, expand if things go well, abandon if they go badly, or switch inputs/outputs. Myers (1977), Dixit & Pindyck (1994), and Trigeorgis (1996) showed that these real options can be valued using the same no-arbitrage machinery as financial options: the project value $V_t$ follows a stochastic process and the decision rule is the solution to a stopping or dynamic-programming problem. The expanded valuation is $\text{ROV} \ge \text{NPV}$; the gap is the value of flexibility.
Option to defer on a binomial tree
A project will generate stochastic present-value $V_T$ if undertaken; the investment cost $I$ is known and constant. Without the option to defer, the NPV rule says invest iff $V_0 \ge I$. With the option to defer, the value of waiting is an American call on $V$ with strike $I$.
Cox-Ross-Rubinstein tree on $V$: at each node, the decision rule is $\max\{\, V - I, \; e^{-r\Delta t}[\,p V_u + (1-p) V_d\,] \,\}$.
In continuous time with geometric Brownian motion, the optimal exercise threshold is not $V^* = I$ (as NPV suggests) but higher:
Gold: ROV (with option to defer). Blue dashed: static NPV $= V_0 - I$. Vertical red line: current $V_0$. Vertical gold: Dixit-Pindyck hurdle $V^*$. Gap = flexibility value.