The definition of a polynode is not indexed, but you can find it by searching the Softimage User’s Guide. Here’s what I found in the 2011 User Guide (I still tend to use the CHM for searching 😉

“…polynodes (also known as polygon nodes or texture sample points). There is one polynode for every corner of a polygon. At a vertex, there are as many polynodes as there are adjacent polygons.”
–Softimage User’s Guide, About User Normals

“There is one polynode for each polygon attached to a point.”
–Softimage User’s Guide, Displaying Polynode Bisectors

Every polygon has a polynode for each of its corners, so at a given vertex there are as many polynodes as there are polygons that share that vertex.
–Softimage User’s Guide, About Vertex Colors

“Each UV pair in a set of coordinates associates a location on an object, called a “sample point”, to a location on an image. On a polygon object, the sample points are polygon nodes, or polynodes. There is one polynode for each polygon corner. ”
–Softimage User’s Guide, About Texturing in Softimage

Here’s a default cube with the NbPolygonNodes and NodeNormal attributes displayed. Each of the eight vertices in the cube is shared by three polygons, so there are 8×3=24 polynodes.