Means for the representation of variability in UML 2.0 interactions, as presented in a previous work, are further formalised and given a mathematically formal semantics. In this way, UML 2.0 interactions can be used in the conception and development of system families within domain and application engineering tasks. Following the transition from domain to application engineering as a configuration endeavour, resolution of the variability according to a given configuration is captured by a denotational semantics for plain interactions extended to the features for the specification of variability. An example based on a previous case study explicates the semantics hereby defined. Key words: UML interactions, variability, system families, product lines, formal semantics