At the opposite extreme from a black box are those completely interactive applications that let the end-user play the role of an intelligent genie who guides the solution search and who intervenes to modify the solution, or even the constraints to get a better solution. How does this kind of application influence the model and implementation for a program using Solver?

In designing the model for such an application, you, the developer, definitely have to distinguish those constraints that are susceptible to user-modification. And certainly the way that the application manages this user-interaction has a direct impact on the implementation and solution procedures.