If you have in a drawmethod more primitivecalls then PrimCount indicates whhat of this calls is catched. For example: Your Drawmethod look like this:
...
device.drawLine(new xyz(0,1,0), new xyz(2, 2, 2));
device.drawPolyLine(xyzarray);
...
if the PolyLine is catched then you retrieves a PrimCount of 1.

See Also

HitItem Class | Minais.Drawing3d.Selection Namespace