The Existing vertex panel enables you to update existing vertex in the selected feature's geometry by coordinates and by GPS. It also enables you to split a polyline feature into two separate features.
This panel can be accessed only from the Editing context menu > Position menu option
Existing vertex panel
East: Displays the current Easting coordinate of the tapped/clicked position as default. Enables you to enter the new Easting coordinate.
North: Displays the current Northing coordinate of the tapped/clicked position as default. Enables you to enter the new Northing coordinate.
Height: enables you to enter the Height of the new vertex that you can save with the [Insert Vertex] button if the the layer can store 3D-coordinate
Modify - updates the vertex with the modified coordinates in the selected geometry
GNSS - takes over the Easting, Northing, Height coordinates of the current GNSS position to update a vertex into the geometry at the current GPS position
Delete - Removes the selected vertex from the selected feature's geometry
Cancel - Closes the panel
Splitting Polyline - Splits the polyline geometry at the selected vertex into two separate features. Disabled when working with point and polygon features.
Height (Z value) can be stored in MAP, SHP, DXF layer file formats. Please also have a look at this tutorial here: http://forum.digiterra.hu/viewtopic.php?f=60&t=302