Expanded PVC sheets are extruded sheets of PVC with a foaming agent added to slightly expand the material, reduce the density of the sheet, and give it an expanded core. This sheet is a solid homogeneous extruded sheet with smooth surfaces, but if you look at the end grain of the material you will see a slightly cellular edge. The purpose of the foaming is to reduce the actual amount of plastic in the sheet. This reduces the cost of the sheet while also reducing the weight. If you are familiar with Azek building material, expanded PVC sheet is a very similar product. It is impervious to water and holds staples and paint well. This makes it a very cost effective material for upholstery substrates. Expanded PVC can also be easily heat formed and glued with PVC cements and fabricated using standard woodworking materials.
The trade off for lower cost, weight, and density is a slightly less rigid material. Thus, in some large seat cushion panels, King Starboard or Starlite are better long term options. Also, the cellular edge of expanded PVC sheet can collect dirt and be somewhat difficult to keep clean.