GGU-3D-SSFLOW: "Individual depths" menu item
You can edit the depths of a node by double-clicking. After selecting a node the depth of the current depth of the active plane is shown in the dialog box.

By entering a different value for z directly you change only the depth of the selected node in the active plane. After clicking "Edit all z" the depth of the nodes in all planes at the selected x/y position can be altered.