Bootloaders used on tinyUSBboard implement specific code, allowing normal firmwares to utilize some of the bootloaders functionality.
This way you can enable your firmware to do things (like writing to the flash), which normally is impossible due to hardware restriction.
This text will be extended in future. For now I will present you the API + example only - more detailed explanations may follow.
by Stephan Baerwolf, Ilmenau 2014