When you use the Show Vertex Normals command, Modo displays small lines that represent the vertex normal facing directions for each vertex in a Vertex Normal map. Split lines represent creased hard edges, and single lines represent smoothed edges. You must have a Vertex Map present to see anything after using this command.
You can create a map based on the object's current smoothing by using the Set Vertex Normals command. The resulting map appears in the Vertex Map Lists viewport.