OpenEmu uses "cores" (based on popular emulators like Mednafen, Genesis Plus GX, and MAME). Not every core needs a BIOS. For example, the NES and Game Boy cores are purely cartridge-based and run via hardware-accurate simulation without a BIOS. However, the following to function:
Use systems that with OpenEmu: NES, SNES, Game Boy, Genesis, etc.
in a BIOS pack means:
Needs the disksys.rom to load disk-based Japanese classics.