In the Virtex-II Pro Data Sheet, (ds083.pdf) version 4.1 from November 04, page 14, Table 22: "RocketIO X Reference Clock Switching Characteristics," the reference clock total jitter pk-pk is stated as 0.012 UI at the different serial speeds.
What is the unit interval (UI) referring to?
The Tgjtt number refers to REFCLK.
The unit interval (UI) is based on one clock period NOT the serial bit period.
If the application is running at 2.5Gb/s, and REFCLK = 125 MHz,
UI = 1/125 MHz = 8 ns
Tgjtt = 0.012 x 8 ns = 96 ps