Second, third, fourth letters: they refer to the matrices of the group variances \(\Sigma_{V}\), the within-group covariances \(\Sigma_{W}\) and the between-group covariances \(\Sigma_{B}\), respectively, by indicating if they are unique (U, i.e., equal within and across components), isotropic (I, i.e., equal within components), equal (E, i.e., equal across components) or free to vary across components (F).