Beginning with Windows 2000, Microsoft has updated the Advanced RISC Computing (ARC) syntax in BOOT.INI to support the SIGNATURE() syntax, which offers a more accurate method for Windows 2000 to boot up in certain conditions. You will see this syntax, which is otherwise identical to SCSI(), if you install Windows 2000 on a hard drive that is larger than 7.77 GB, or if the installation occurs past cylinder 1024 on that drive. You can also see this syntax if the BIOS on your SCSI controller is disabled.
So what does it do? On an IDE system where the BIOS doesn’t support INT13, the SIGNATURE() syntax allows Windows 2000 to find the correct drive mappings, regardless of how your SCSI controller numbers the drives. This is required for Windows 2000 because it is a Plug and Play operating system that must interoperate with disk controllers a certain way, so it could conceivably get incorrect information from the BIOS in a SCSI controller.