I’m assuming you’re referring to the eTool Default database. It’s just what’s available in the material library based on industry averages. There is an option for ‘PVC frame with steel bracing’ but that’s pretty much it if you want to use a ready-made option from the material library. You could use the ‘steel > general > unspecified’ material just for the frames (if you know the quantities) if the other options are not suitable.