Once the initial design has been produced, the goal is to test the flexibility plan and understand how the production and costs change as a result of deviations seen in implementation. A real system will always have some variations and small problems. We want to design the system to be robust to these hiccups so that a small problem does not kill the productivity and blow up the part costs.
For example, what if you have a part which is in high demand one week but has no demand a different week? How will the system operate? If this part was flexible, it was helping fill up the machines to keep them busy because it could choose to go to an underutilized machine. Are there enough other parts that are flexible to cover the change in demand?
Cost/piece is also sensitive to the flexibility plan. How much of the cost is due to overcapacity? How will the station efficiency impact the cost/piece? Typically, we see that costs are very sensitive to efficiency, and this is the primary justification for budgeting and preparing a detailed operations plan to manage the efficiency. In our experience, a common cause of project failure is improper accounting of the relationship between cost and efficiency; see any of the material about lean accounting for more information.
Currently, SeedTactic: Planning has no dedicated support for sensitivity analysis, but the good news is you can perform it anyway. For example, you can go to Stage 1 and update the part quantities and then flip forward to the projected operations to see how the results change. Or you can change the station efficiency and examine how the part costs change.