This paper considers uncertain systems from a behavioural point of view defined via quadratic differential forms. This uncertainty definition is closely related to the integral quadratic constraint uncertainty description commonly found in robust control theory. The paper presents a frequency domain condition for the set of behaviours of a given uncertain system to contain the set of behaviours of another given uncertain system. This result is useful in uncertainty modelling problems in which one wishes to consider the trade off between model complexity and model conservatism.