As we are going to create FAST diagram, the concept describes the method being considered, or elected, to achieve the basic function(s).
The concept either represents the current conditions (as is) or proposed approach (to be). It is always good to create a “to be” rather than an “as is” FAST model, even if the assignment is to improve an existing product. The “to-be” approach provides an opportunity to compare the ideal product to the current product, and thus resolves
questions on how to implement the differences.
An “as is” model will limit the team’s attention to incremental improvement opportunities. To trace the symptoms of a problem to its root cause and to explore the ways to resolve a problem, an “as is” model is extremely useful. It is useful because of the dependent relationship of functions that form the FAST model.