fastboot3DS is a firmware loader for the Nintendo 3DS. It works like boot9strap, but offers additional features such as:
A menu that can be accessed by holding (Home) on boot to perform certain actions
The ability to chainload payloads directly from fastboot3DS itself (instead of through Luma3DS)
For example, you could configure it so that holding multiple buttons at once boots you into a payload
The ability to display a splash screen on startup, before Luma3DS has booted
The ability to backup / restore your NAND, as well as backup your bootroms and otp.bin
The latter can no longer be done with boot9strap as of boot9strap v1.4, but can still be done through GodMode9
Compatibility with HOME Menu FIRM payload shortcuts, such as this one for open_agb_firm
Our guide installs boot9strap since it's what most end users will be fine with, but if you're a power user, then you may be interested in installing fastboot3DS.
While the installation of fastboot3DS is a generally safe process (equally safe as installing boot9strap), there is still a chance that something goes catastrophically wrong. Installing fastboot3DS (just like boot9strap) involves writing to the 3DS's FIRM partitions, so if something goes wrong, you may need ntrboot to fix it.
Troubleshooting may differ for some situations (generally for things relating to FIRM-based payloads) when fastboot3DS is installed, so you should disclose that when getting help with such a situation.
