On various BIOS-based systems, the reserved partition would contain files necessary for booting windows from its system volume, bridging the gap between what could be accessed by simplistic MBR boot code, the NTFS boot code block, and the NTFS-understanding, ARC emulating (for NT5) or EFI-emulating (NT6) boot system that would load target system.
Details on whether reserved partition would be created and what would be on it depend on hardware you're installing on, and if separate boot partition is necessary windows installer would inform you about need to create an extra partition.
Details on whether reserved partition would be created and what would be on it depend on hardware you're installing on, and if separate boot partition is necessary windows installer would inform you about need to create an extra partition.