PROD

Denotes if the firmware image should be used on a production mode device or not. This is set to 1 on personalized + release firmware images (complete images that have ECID included and such), while set to 0 on yet-to-be-personalized images. This is probably to prevent production devices from running them, since Apple wants iTunes to phone home for every restore, but at the same time will not affect their debug devices, because there is a check in iBoot's do_load_image(); routine that will skip all tag checks if it is running on a debug device.