Selecting Ref Clock for Transmit or Receive Path indicates the default setup that is passed to GT Wizard reset state machines. You can also select different reference clock in application by programming Video PHY configuration registers and apply reset in appropriate paths.
Figure 1. Vivado IDE – DisplayPort Protocol with GTXE2 Transceivers
Note: Setting QPLL as the TX PLL type and CPLL as the RX PLL type is the valid configuration of the Video PHY Controller driver, based on the DisplayPort configuration and it is not recommended that you change the TX/RX PLL types. Hence, it is grayed out in the Vivado® IDE customization screen.