Primary partitions are marked from 0 to
3 (hd?,0), (hd?,1),
(hd?,2), (hd?,3). Logical partitions in the extended partition are counted from
4 up, regardless of the actual number of primary partitions on the hard disk,
e.g. (hd1,7).
As the name implies, GRUB passes the control of the
boot sequence to another bootloader, located on the device to which the menu entry points. This can be a
Windows operating system, but also any other, including Linux.