Conceptually, we can continue to build the function model by adding functions in the WHY and HOW directions, and expanding those functions in the WHEN direction. The modeling process is considered complete when the VE Team agrees that the model reflects the problem or opportunity to be resolved. This is illustrated by adding vertical "scope lines" to the function model, as shown in Figure

The area between the scope lines graphically describes the functions and their dependent relationship under study. The function(s) to the right of the right scope line shows the input functions or functions related to but outside the study scope. The function(s) to the left of the left scope line describes the highest order functions or the objective(s) of the study. The functions to the immediate right of the left scope line are defined as basic functions. Functions to the right of the basic function(s) are considered secondary, or supporting functions (refer to the description of basic and secondary functions above). The horizontal (X-axis) function path that joins the basic function is called the "Major Critical Path". If a function along this path is disturbed it will significantly alter the way the basic function(s) work. Other horizontal function paths are called "Minor Critical Path(s). Changing the functions on these paths will change attributes, features, or support functions, but would not significantly alter the basic function.
After placing the scope lines on the function model the team should relate the model to the problem statement and study objectives to assure comparability.