Difference between cpu backplane ad io expansion backplane

[Milosh 0]

What is the difference between these two bacplanes?

Can we mount cpu to io expansion backplane?

[BobB 58]

No. That is why there is a CPU backplane.

[Michael Walsh 268]

The expansion backplanes have a slot for the power supply only (No CPU slot).   There are extra connections on the expansion backplanes to connect the cable to the CPU backplane and additional expansion backplanes.  See the drawing below: