Due to an error in the Intel® Cyclone® V Avalon®-MM Interface for PCIe* Solutions User Guide, you will see a reference to the VCCA_GXB voltage rail which doesn't exist on Cyclone V devices.
The "Making Analog QSF Assignments Using the Assignment Editor" section of the user guide provides instructions for setting the VCCA_GXB power rail.
However, Cyclone V devices do not include this rail.
You can safely ignore these instructions.
This is not scheduled to be fixed in a future version of the user guide.