When I say 'the net boot process', I mean the act of loading the image into memory, setting up any tables, up until the jump to the required location in the image.
The net booting program executes the net boot process. The net boot program may be a rom, but not neccassarily. It is a set of instructions and data residing on the booting machine.
The image, or boot image, consists of the data loaded by the net boot process.
When I say 'the PC boot process', I mean the general PC rom bios boot process, the setting up of hardware, the scanning for adaptor roms, the execution of adaptor roms, the loading in of the initial boot track. The PC boot process will include the net boot process, if one is present.
When I say client, I mean the PC booting up.
When I say 'image host', I mean the host where the boot image is comming from. This may not have the same architecture as the client.
The bootp protocol is defined in RFC951 and RFC1084. The tftp protocol is defined in RFC783. These are available on many sites. See Comer 1991 for details on how to obtain them.
A bootp server is the machine that answers the bootp request. It is not neccessarily the image host.
"Can" and "may" means doesn't have to, but is allowed to and might. "Must" means just that. "Cannot" means must not.