Michal Necasek investigates a few unexplained bytes in the boot sector and also some bytes in
The first person to make this discovery (as far as we can tell) was Daniel B. Sedory aka The Starman, whose illustrated PC DOS 1.1 boot sector page is much nicer than anything I could put together.
Perhaps see also Sedory’s page
The Mystery Bytes of the Windows 95 MBR (paraphrased)
via Yellow Plastic