Edwin Pawlowski wrote:

Styrofoam is the blue material extruded by Dow Chemical. Styrofoam is a
registered trademark of Dow but often the name is mis-used when referring to
any other type of polystyrene foam board, extruded or molded and wire cut
such as the CLOSED cell expandable polystyrene board, sometimes called bead
board.

EPS is open cell. X (extruded) PS is closed.

Nick