If you're looking for a complete "paper" or detailed documentation on handling these files, I recommend consulting the following resources:

These three files— bios-cd-u.bin bios-cd-e.bin bios-cd-j.bin —are the foundational firmware images required to emulate the

Sega CD games are region-locked. A US BIOS cannot natively boot a Japanese game. Having all three ( u , e , j ) ensures you can play any game in your collection.