The functional ANOVA expansion of a multivariate mapping plays a fundamental role in statistics. The expansion is unique once a unique distribution is assigned to the covariates. Recent investigations in the environmental and climate sciences show that analysts may not be in a position to assign a unique distribution in realistic applications. We offer a systematic investigation of existence, uniqueness, orthogonality, monotonicity and ultramodularity of the functional ANOVA expansion of a multivariate mapping when a multiplicity of distributions is assigned to the covariates. In particular, we show that a multivariate mapping can be associated with a core of probability measures that guarantee uniqueness. We obtain new results for variance decomposition and dimension distribution under mixtures. Implications for the global sensitivity analysis of computer experiments are also discussed.of Beckman and McKay is possibly the first work discussing the stability of sensitivity analysis results for perturbations in the model input distributions [6]. As Saltelli et al. underline, the use of multiple distributions may controversial [73]. Nonetheless, it has become a de-facto part of several studies and is frequently adopted.Our purpose is to offer a systematic investigation of the impact of removing the unique distribution assumption on the classical functional ANOVA expansion of a multivariate mapping. We consider two paths that emerge from current and past practices. The starting datum is that the analyst posits a set M = µ 1In the first path, the analyst evaluates the model for each distribution in M separately and obtains sensitivity measures for each distribution -without-prior path, henceforth. In the second path, the analyst assigns a prior over the the distributions in M -with-prior path henceforth. For each path, we investigate six relevant notions: existence, uniqueness, orthogonality, monotonicity, ultramodularity, variance decomposition and dimension distribution. We study the implications in light of three sensitivity analysis settings: factor prioritization, trend identification and interaction quantification.Let us report some of the findings. In both paths, existence is ensured if all the posited measures are compatible with the functional ANOVA expansion of the input output mapping. Regarding uniqueness, in the without-prior path the analyst is dealing with as many functional ANOVA expansions as many are the cores in M. A core is defined as a set of probability measures that lead to identical expansions. Thus, one has uniqueness if all the posited measures belong to the same core. In the with-prior path, one regains uniqueness: a multivariate mapping can be uniquely represented as the mixture of functional ANOVA expansions. Regarding orthogonality, in the without-prior path it is preserved. In the with-prior path, mixtures of classical functional ANOVA effects are not orthogonal with respect to the mixture of the distributions in M. Regarding monotonicity and ultramodularity, in the without-prior path t...