What a 'flash script (.fsc)' is:
http://www.flashtool.net/fsccreate.php
Building one is 'close to impossible' for non-skilled person (I dug a bit about it with the USB logger with no success - I am not a skilled one )
The downloaded image shall therefore have this .fsc file included. With XperiFirm I found a version with MM for my phone that included the flash script and flashed with no hassle back to Android 6.01 using that one (F5121_34.0.A.2.292_R4A_Customized IT)