There is actually 1 rule to find the correct ruu.
Fastboot getvar version-main
That command tells the number of the ruu you need as a minimal number to start with. And in our case it has to start with endevouru because it's the codename for our hox.
So if your number is 3.14.401.31 you need a ruu that matches that exactly.
You can however flash different ruu's but it has to have a higher number. For example :
2.17.401 = main number, that number can go a step forward and flash the 3.14.401 ruu.
Obviously you can only downgrade with a ruu if you have s-off. So someone who is on 3.14.401 and a s-on device can not install the 2.17 ruu because it tries to downgrade the bootloader.
Hope this answers your question a little bit.
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