@KaiJax 

When doing a mixed system only the units capable of a feature will make use of it.  This would mean that if you were to using wireless as a backhaul only the 2 XE75s would make use of 6GHZ.  Backhaul traffic between WiFi 6 and Wifi 6E would be over the two bands they have in common 2.4 and 5GHz unless you have disabled one of those bands.  However, if you have a switch and Ethernet cables ran your best option would be to use ethernet backhaul for all units.  Feed the 3 satellite units into the switch and then connect that switch to the main unit.  This will in most cases give you the best backhaul and make unit deployment easier.  Additionally, its less traffic down the useable bands so client performance will be better.  

You could even open 6GHz for clients if you have any that can use 6GHz 

 Thanks Carl.

Last night I set up my system like this: two satellite deco, XE75Pro and W7200, are connected to the main deco

unit (XE75 Pro) via ethernet cable. Another satellite deco W7200 is connected to main deco wireless. When I

open deco app and found that this deco (wireless backhaul) shows connection frequency 2.4GHz and 5GHz_1.

Since this is a triband router, does this mean 5GHz_2 is used as wireless backhaul?