Hi @aaidaa,
Have you read the documentation related to Construction of Desirability Functions ?
"The individual desirability functions [...] are smooth piecewise functions that pass through three defining points. These points are called control points (Low, Middle, High) and can be used to interactively control the shape of the desirability function."
The optimization algorithm choice is also explained depending on the factors type and presence of constraints.
Hope this answer may help you,
Victor GUILLER
"It is not unusual for a well-designed experiment to analyze itself" (Box, Hunter and Hunter)