The main instruction set of the baseband. You can get these files from /usr/local/standalone/firmware on the corresponding firmware's ramdisk.
The EEP files is the external EEPROM file. The FLS is the firmware.
The iPhone 4 has a single baseband firmware file. For example, the 4.0 baseband firmware filename is ICE3_01.59.00_BOOT_02.06.Release.bbfw. This is actually a .zip file which contains four baseband firmware files. As of iOS version 4.2.1 the baseband version is now checked by iOS.