EC = Embedded Controller
It's a silicon chips used for low level power control in computer systems.
EC Firmware, the software who really control the EC working, it works independent to the operation system.
EC Firmware Update, update new EC firmware which include bugfix, bugfix, bugfix and bugfix...
Sometimes OS will require specific EC firmware version to implement some special method, but that is another case.
Because EC firmware update means there is bugfix or control method update in it.
That's why PA_CUS1 did not include these files really make me confused.