This is a fan-made hack applied to the ISO that restores "beta" content cut from the final game due to time constraints. Applying this patch to your adds:
An ISO file is a digital replica of the original CD-ROM. The original RE3: Nemesis shipped on two discs for the PlayStation (though the second disc was mostly bonus content and a save data transfer for Dino Crisis ). In the context of retro gaming, an ISO allows you to play the game on hardware it wasn't intended for, specifically: Resident Evil 3 Nemesis Iso