Hi,
from the X2X slices point of view, it’s exactly like with every other bus controllers: the’re attached to the right side of the controller.
The difference between those 2 controller setups on the picture is only, that the extended bus controller backplane has a slot also on the left side to plug an additional fieldbus interface of type “X20IFxxxx-1” (for example a CanOpen interface).
So if you don’t need a additional fieldbus interface reachable via the Ethercat slave, the left configuration is enough to control X20 slices via Ethercat.
Best regards!