If you select Edit, Master Files, Material Types, you are presented with the table below.
And if you select Edit, Master Files, BW Elbows, you get the following:
You can see that a pipe bend is made of Pipe Material, while a BW elbow is made of Buttweld material. Similarly, flanges are assumed to be made of Forged material. If you need the materials for flanges and forged fittings to be different, you can alter the master files so that flanges use material E rather than material F. Be aware that making this change will affect specs you have already built. You may therefore want to consider making this change to a copy of the Data folder placed under a client folder, so that it is used only for jobs under that particular client.