This field is from the partition table. It is known as the OS Flag, the Partition Type, Filesystem Type, and various other names.
There are many possible values defined by various vendors. Some particular values of interest include:
0x00 Unformatted partition. 0x01 FAT12-formatted partition. 0x04 FAT16-formatted partition < than 32 MB. 0x06 FAT16-formatted partition >= 32 MB. 0x07 Partition formatted for use with an installable filesystem (IFS) such as HPFS or JFS under OS/2, or NTFS under Windows NT. 0x0A OS/2 Boot Manager partition. 0x11 FAT12-formatted partition < 32 MB which has been 'hidden' by Boot Manager. 0x14 FAT16-formatted partition < 32 MB which has been 'hidden' by Boot Manager. 0x16 FAT16-formatted partition >= 32 MB which has been 'hidden' by Boot Manager. 0x17 IFS-formatted partition which has been 'hidden' by Boot Manager. 0x35 Partition assigned to an LVM volume. 0x82 Linux swap partition 0x83 Linux partition 0x87 HPFS386 Fault Tolerance mirror partition, or NT stripe/volume set. 0xC7 Disabled HPFS386 Fault Tolerance mirror partition.