Avaya has not published an official document to Fabric Connect so far.
We can only spaculate how Fabric Attach is implemented from Avaya...
I would assume that at the end of the day you have with FA a mechnaism to connect non SPB
capable devices to an SPB core. To make it work you need a FA software on your SPB switch wich
they have now introduced and also a sofware for your non SPB switches like
e.g. ERS5k or ERS3k wich supports FA.
For a complete new deployment with SPB , you can choose only SPB capable devices.
I think FA is more interesting for networks wich have a large number of older non SPB capable devices deployed , so you can upgrade your core and still can get some of the SPB benefits for your legacy switches.
But I am completly speculating here, we have to wait for some official docs from Avaya to get the full picture.