A Feature Column mapping is defined by its Feature Name, which should normally be the name of a valid feature (attribute or reference) of the element which appear in the tables' lines. If a line represents a semantic element S1 but you want on some column to show a property of an element reachable from S1 instead of S1 itself, you can use the Feature Parent Expression ( Advanced category), which is evaluated in the context of S1 and should return an element S2. S2 will be considered the target element of the cell at this intersection instead of S1.
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The Label Expression is used to compute the text to show in each cell. It is evaluated for each cell in the context of its target semantic element. If you compute text for elements whose EClass have no common EStructuralFeatures (inherited or with same name), you can use * as Feature Name to skip the feature name validation during the cell creation.