The dialog assumes the shape shown in the following image after the Bars - beams option is selected from the Functions selection list in the Diagram definition dialog.
The dialog is unavailable for 2 structure types: 2D truss and 3D truss.
At the bottom of the dialog, you can select a single node or a selection of nodes for which a function will be formulated. The function will be generated for a point on a member defined in the Position field (a position of such a point can be defined in relation to member length).