A couple of specification and modelling languages dedicated to the particularities of mobile systems have been developed as mobile hardware and software gain wide acceptance. Though these languages facilitate describing systems of mobile objects at different levels of abstraction, they do not allow to distinguish between the ever-mobile units and those which provide the fixed subsystem as the context for the mobile entities. However, as this is an essential distinction for many scenarios, in this article we present some suitable constructs as an extension of an already existing specification language and illustrate their use with an example from the area of information retrieval.