The 100MiB partition is your system partition. Your so-called "real stuff" volume is called the boot volume in Microsoft parlance. Microsoft puts Microsoft Boot Manager, the BCD database, and a few other things into the system partition. These, too, are "real stuff". The essence of a system volume is that it is dedicated to the machine as a whole, independent of which particular operating system is used. Hence its name.
Windows NT has in fact never installed in terms of just a single volume. It always installs in terms of two. This has been the way that Windows NT has always worked. It isn't DOS.
Non-x86 flavours of Windows NT have always, going back to Windows NT version 3, had (and installed to) separate boot and system partitions by default. The system partition is where the boot loader program lived, in a file on that partition. There are no bootstrap programs in MBRs and VBRs on such platforms. The Windows NT boot loader program is loaded from file and run directly by the machine firmware. On ARC machines, for example, the Windows NT 5.0 bootstrap loader was an ordinary file
osloader.exe on the ARC System Partition.
On x86 platforms, in contrast, up until Windows NT 6.1 ("Windows 7"), the installation default, for a blank disc, was to still have boot and system partitions, but to combine them into one, and not use the designated partition types for true system partitions. (For installing to a non-blank disc, containing a prior version of Windows NT, one actually gets separate system and boot partitions, as the new version of Windows NT adds its own boot partition but re-uses the existing system partition.) Rather, the combined boot+system volume would be a Microsoft Data partition acting as a Poor Man's system partition, and this would be alright because the firmwares on x86 systems (up until roughly 2005) didn't mandate the presence of true system partitions.
From Windows NT version 6.1 ("Windows 7") the installation default for x86 is now to always create separate boot and system partitions, even when installing to blank discs. So now the behaviour is the same for x86 as it always was for non-x86 and things are uniform. The x86 world is finally coming into the fold. There now exist x86-64 systems (at least) with UEFI firmware, that boot Windows NT 6.1 in the same way that Windows NT 3.1 bootstrapped on ARC machines all of the way back in 1993, using a true system partition, with no VBRs or MBRs at all.
On x86 machines with old PC/AT and PC98/PC99 firmwares, the startable partition, for Windows NT going back to version 3, is always the system partition. (In the old PC/AT and PC98 boot process, there was a bootstrap program in the MBR that looks for a "startable" (a.k.a. "active") volume, and there was a second bootstrap program in the VBR, of that volume, that is loaded and run by the first bootstrap program.) The system partition is always required to be marked "active". (Be aware that this is not how it is identified.) The system partition's VBR contains a bootstrap program that:
- on Windows NT up to and including version 5.2 loads the Windows operating system loader,
- on Windows NT from version 6.0 ("Windows Vista") onwards, loads the Microsoft Boot Manager,
BOOTMGR, which in turn loads the Windows operating system loader,
WINLOAD, from the boot partition
The operating systems that people use now are not like the ones that they used to use. Windows NT isn't DOS, and isn't even like DOS. Cast off the DOS Think. Windows NT's design philosophy includes some ideas about operating system bootstraps that are based in the world of the high-end workstation and server rather than the world of the personal computer. For a few years, because of limitations of x86 personal computer firmware, it had to ape DOS a little bit, but now that the x86 world is finally catching up in terms of firmware capability, how Windows NT is designed to work, and has really worked for some eighteen years at this point, is starting to shine through even in the personal computer world.