To define the projection line select the Define Projection Lines command (21M3). The command defines the line in the layout, the projection of which can be later shown in a longitudinal profile and cross-sections. The label is used as identification when transferring data from the layout drawing to longitudinal and crosses-sections.
First draw the polyline (line, polyline, 2D polyline, or 3D polyline) in the situation and define its elevation. Press the command 21M3 and select the borders lines. In the command line define the line name. Confirm with Enter.