Hi Daniel.
If you see, maximun port capacity por Aruba 5406R is reached with 12 40GE ports. In that situation, your routing/switching capacity is: 12x40Gbps = 480 Gbps, and considering in/out direction (full duplex) you get the 960 Gbps of the datasheet. So your switch needs a minimun switching capacity of 960 Gbps to work at wire-speed. It seems that switch fabric speed is the internal swithing structure of the switch (not available to public, only to Aruba HPE personnel) that is used to serve that minimun switching capacity.
You need 960 Gbps (that depends on the physical port configuration of your switch) considering the mentioned port configuration, and the switch has 1015 Gbps to serve that demand (switch fabric speed). If we suposse there is no other routing/switching demand, the switch has 55 Gbps to serve a higher demand.
I hope we have another extra input about this info in the datasheets.
Regards