In short, it is Z in the typical electronics sense.
The extended version from our document
Z=Float. The PMODE pin can be made to float either by leaving it unconnected, or by connecting it to an FPGA IO and then configuring that IO as an input to the FPGA
We were able to pull with 10K for the Non HiZ pins and it seemed to work. I wish they would characterize it, but I believe their superspeed explorer kit uses similar pullup or pulldown resistors.