The default right mouse button option, Select pairs, allows you to select two points, circles, circular arcs, ellipses, or elliptical arcs. The Concentric constraint will force selected points, or centers to be at the same location. For circles, circular arcs, ellipses, or elliptical arcs, they do not need to have an actual center point. The right mouse button option, Select multiple, allows you to select a continuing series of points, circles, circular arcs, ellipses, or elliptical arcs. In the series, after you have selected two edges, a constraint is created and then the second edge you selected is used as the first edge for the next pair. This continues until you either use the right mouse button to start a new series or return to standard pairs selection. A series of these may be preselected before selecting the function and they are treated like a series selected in 'Select multiple' mode.