Virtex data sheet uses R and r to represent Vref pins in the package drawings. What are the differences between R and r?
If an IO_STANDARD does not require the use of Vref, the IO_VREF sites can be used as IOs.
The R indicates that these pins are always IO_VREF, regardless of the device size. The r indicates that these pins are IO_VREF for some devices but not all, and in the smaller devices these are just IO.
If designers want to migrate a design from a smaller device to a larger device in the same package, AND they are using an IO_STANDARD that requires a VREF source, all R and r pins should be connected to the same voltage source in the larger device. They should use a PROHIBIT constraint to prevent an IO from occupying the IO_VREF sites when doing the design in the smaller device.