It dictates whether the system behaves as an NTSC-U (USA), NTSC-J (Japan), or PAL (Europe) console.
The PS1 BIOS is region-specific. If you try to play a Japanese game (NTSC-J) using a USA BIOS (NTSC-U/C), some titles may glitch, run at the wrong speed, or refuse to boot. That’s why serious emulation setups include multiple BIOS versions. ps1-rom.bin bios