This article discusses software internally used by Apple.
Acquiring a copy without Apple's consent is illegal and may result in being scammed.
This is a documented list of Internal Firmware types:
- InternalUI Builds, which are Development/Debug builds of *OS
- NonUI builds, which are for Factory and Development testing of *OS devices.
- CarrierOS, which are meant for Carrier Testing.
- Combo UI, which is a combination of InternalUI and NonUI
- Horizon Firmware, which are stripped-down NonUI builds to recalibrate parts for iPhones.