Assigns (different) shapes for each attribute value of one nominal column, i.e. for each possible value. Supporting views then render datapoints with the shape associated with the corresponding attribute value. If there is for example a dataset with two different classes ("class1" and "class2"), "class1" may have a circle and "class2" a triangle assigned. When looking at the dataset the values can easily be distinguished through their shape.
In the dialog the nominal column with the possible values may be selected. The possible values appear in the left column and the shape can be set in the right column of the table by clicking on it and selecting the desired shape.