They act as reflective surfaces that emit light. You will almost always find them in windows or above broken roofs. They are used to reflect light into dark areas. They are a PITA to find sometimes as they are only visable from one side. It's much easier to find them in wireframe mode sometimes.
They ues their own special set of materials that can be found in the FX_Atmospherics package.
They ues their own special set of materials that can be found in the FX_Atmospherics package.