The idea of a multiboot system is that you can run any distribution on your device. You initially boot a rootfs as the main system and then to boot quickly (bypassing the bootloader stage) into different distros/ROMs as subsystems. New kernels are loaded on the spot with kexec.
You can switch between kernels and you can run the system from internal disk, CF, SD or NAND. This means that you can run kernels such as Yonggun, Poky, Angstrom; ROM images such as Cacko and Sharp; also pdaXrom and Android. Restrictions apply; for example, the Sharp ROM has the file structure hard coded.
Note that Angstrom has the altboot utility that also allows multibooting.
- This is a summary of ZDevil's How to thread