It is a spare 256k bios that will help you fix a corrupted bank if you ever run across it. The X3 has 2megs of bios space broken up into 8 banks of 256k. If you loaded the X3 bios then you would have used 1meg of the total amount and most likely placed it on banks 1-4.
If you are wanting to experiment with a different bios you can load one on the other free banks. Or if you choose to load another 1meg bios, like a different version of the X3 then you would need to load it on banks 5-8. Here are the switch setting for the one meg banks. If you want to utilize 256k banks then you can search for the switch settings... they are on the forums.
1meg Bank1 = on, on, off, off.
1meg Bank2 = off, off, on, off.
