Enjoy Hello,
Will try to answer your question.
There are two different type of firmware that release by Vendor:
1. User/Shipping Firmware = Firmware that release after device is ready to sell in retail store. Mostly this is stable version, and secure ones.
2. Factory Firmware/Engineering Firmware = Firmware that only available at production purpose in vendor factory. This firmware is not available for user, this kind of firmware is for developer and testing purpose only. Mostly this firmware is AOSP with minimal GUI, all debug port and feature is on, and some is already root. This firmware can be usefull for repair purpose, like restore imei, etc.
