There are two ways to install BootMii, on Boot2 or as an IOS.
The Wii has two "boot sectors" for a lack of a better word, boot2 and boot1, now older Wiis have a boot1 that can be hacked, if the Wii has a hackable boot1 bootmii can be installed as it's meant to be to boot2 and will load before all Nintendo stuff loads up.
Installing Bootmii to Boot2 makes it immune to pretty much everything there is and it can load even if the rest of the NAND, the internal flash memory is a complete and utter mess, this basically makes your Wii 100% brick proof.
Unfortunately your system has the new boot1, the ones that are so far unhackable, you can still install bootmii but only as an IOS, as a program running from the Wii only after everything else by Nintendo loads up, you can still use bootmii and use some of its features, you can still backup your "wii", the NAND, but in case of a brick, a system update or anything bootmii will go too where as if installed as boot2 it's immune to all that stuff.
So, bottom line
Boot2 - runs before anything else loads, is immune to all modifications of the internal system memory, the NAND
IOS - runs after all the Nintendo stuff loads and isn't immune to anything, it's the same as any other channel or thing installed to your Wii
