We claim that the role of the decision maker in a DSS is to control a heuristic search. In this context, the models involved in a DSS naturally appear as evaluation functions driving the end-user control. This interpretation, which seems to be natural for P. Humphreys' (1989) lower representation levels, highlights the links between models and control and explains the prominent role of the "what if" concept in the DSS literature of the seventies. The focus now however, is rather on modelling, and will turn to the control which is related to flexibility. Hence, our analysis paves the way to the design of more flexible knowledge-based or "intelligent" DSSs. We moreover examine the difficult question of the decision support for higher representation levels.