Greg,
xSD is not a replacement for Fablink. Fablink is used to create manufacturing data in Panel form for PCB Layouts.
xSD is an enhanced version of xDX Designer for schematic design of Multi-Board Projects. It allows for the design inclusion of many boards in a system, including the interfaces between the boards.
Here is a link:
Multi-board PCB Systems Design - Mentor Graphics
Thanks,
Vern