| | In 3D modelling, a normal is the 'facing' of any given mesh or subpart of a mesh. In general, normals are associated with facets and vertices, though the concept associates to much more complex shapes as well (albeit with more math involved).
Strictly speaking, a normal is a vector perpendicular to a surface.
This definition does not make any sense for a vertex, which has no surface. Rather, a vertex normal is defined by taking the average of the normals of the facets in which the vertex takes part. As a result, a vertex which does not take part in any facets (i.e., an isolated vertex) cannot have any logical normal (though any vector can be associated with it as an assigned normal).
Normals are used to determine the angle at which light bounces off of a surface, and thus how much light illuminates the surface. As you can see, this can be considered one of the most important fundamental concepts in 3D modelling.
When normal information is not present, a facet's normal is determined by the winding order of the vertices that make it up. Since Poser disregards normal information, it can only determine a normal in this manner. However, Poser also disregards normals when it renders, treating all materials as {{2-sided materials}}, thus using both the normal and inverse normal as determined by the winding order.
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