Thats a pair of 6xQSFP28 adapter that connects to the FPGA transceivers, with 100Gbps Direct attach cables.
Aka as 6x100Gbps worth of interconnect between two more typical Virtex development boards.
So, is it better or worse than connecting those cards through the transceivers connected to the PCIe and plugged into a PCIe passive backplane? What are the trade-offs? According to the Xilinx docs that card has twice as many xcvs hooked to the PCIe edge than to the both of QSFPs.