mkinitrams uses plugins to find root file system. Plugins scan /dev entries, assemble RAIDs, activate volume groups, etc... Here is the full plugin list:

• dmraid - aka ATARAID, BIOS assisted software raid found on many modern motherborads
• mdraid - Linux Software RAID
• lvm2 - Linux Logical Volume Manager
• EVMS - Enterprise Volume Management System
• CDROM
• splash? - support for fbsplash boot splash themes
• raidify? - on-boot raid conversion

Got exotic setup? - write your own plugin! Its really simple, usually about 5-10 lines of bash code. See Plugin Development on how to do it.