For stiffened panels, when HyperSizer writes out the PSHELL card, the thickness used is not a physical thickness, rather a thickness of exactly one inch is used. See this forum entry for details on how we represent the stiffness: http://hypersizer.com/faq/index.php?topic=115.0
The reason we don’t use density is because it would not be meaningful when used with this non-physical thickness. Remember that in the FEA representation, we are not putting out the individual objects like the facesheet, web or flanges, we are putting out stiffnesses that smear all of these together. Therefore there is no one density that would represent the entire thing, that is why we put it out as a NSM which is just a weight per unit area applied to each element.
When we export PCOMP/MAT8 cards to represent laminates or sandwich panels, we actually do use the material densities on the MAT8 cards and no NSM on the PCOMP cards because in this case the density is a real physical entity.