From "The Software-RAID HOWTO" (http://www.tldp.org/HOWTO/Software-R...O-7.html#ss7.2
Autodetection allows the RAID devices to be automatically recognized by the kernel at boot-time, right after the ordinary partition detection is done.
"This requires several things:
1. You need autodetection support in the kernel. Check this
2. You must have created the RAID devices using persistent-superblock
3. The partition-types of the devices used in the RAID must be set to 0xFD (use fdisk and set the type to "fd")"
Why would you want this? See http://www.tldp.org/HOWTO/Software-R...O-5.html#ss5.9