Methods supporting designers' cognition in early stages are critical. Function modelling in that can be important because it allows understanding the design perimeter as well as communication between engineers. Moreover, in complex system design, no one designer or design team is able to explore and define the design perimeter alone. In order to understand the advantages as well as the limits of modeling approaches we have chosen three approaches Function-Behavior-State (FBS) [1], Functional representation to support idea generation [3] and Affordances [2] and we look at them also in relation with a process on problem definition of complex systems used in industry [3]. Some of the advantages as well as needs for future developments in the case of complex system design are discussed.