With mknbi you can create tagged images for Etherboot. Tagged images are data files, which contains the necessary files for booting up (kernel+root for Linux, kernel+minifs for DOS, ...) bundled together with a special format.
These tagged images are downloaded and understood by Etherboot and Netboot during the boot process.
This package should be used on the server and not on the client. It doesn't contain code which handle the network card, download the image, etc.