Per-Vertex normals represent the direction perpendicular to a surface at a vertex location. They are often used in graphics to produce smoother lighting effects than using triangles directly.
en.m.wikipedia.org
Vertex normal
In the geometry of computer graphics, a vertex normal at a vertex of a polyhedron is a directional vector associated with a vertex, intended as a replacement to the true geometric normal of the surface. Commonly, it is computed as the normalized average of the surface normals of the faces that contain that vertex. The average can be weighted for example by the area of the face or it can be unweighted. Vertex normals can also be computed for polygonal approximations to surfaces such as
I don't recall under what condition they may not be present in Structure SDK.