PROD

The PROD (PRODuction) tag is a bit value that denotes if a firmware is for production devices or not. When set to, the firmware is designated to be installed onto production devices. When set to, the firmware is typically incomplete and designed for internal devices.

This is most likely to prevent production devices from running internal firmwares as Apple wants iTunes to "phone home" for every restore, but at the same time, not affect their internal debug devices. This is acomplished through iBoot's  routine that skips all tag checks if the PROD tag is not set.

C-pseudocode for
if (flags.prod) do_load_image;