The relation between subjective ideas (mental models) consumers have about the functionality of an Advanced Vehicle Control System (AVCS) and the functionality as it is objectively realized (technical model) is analysed. Based on this the correlations between comprehensibility, predictability and controllability and the functionality of the system are discussed as topics which should be respected in the functional specification procedure. And finally in this analysis of the importance of mental models consequences for system testing are pointed out especially stressing the need for an assessment of reasonable and expectable consumer behaviour (product liability!) by a long term, non-obtrusive measurement approach.