VREF group Definition
A group of I/O pins associated with a VREF pin that
                  are grouped for the purpose of specifying voltage. Each
                  VREF group is associated with a specific I/O bank, and
                  all of the pins in a VREF group must use the same
                  VREF voltage.
               
The format of a VREF group location is
                  VREFGROUP_B<number>_N<number>,
                  where B<number> represents the I/O bank
                  number and N<number> represents the VREF
                  index within the I/O bank. For example, the 
                     Quartus® Prime
                  Settings File (.qsf) setting for a VREF group
                  assignment is:
                     set_location_assignment VREFGROUP_B3_N2 -to clock
For supported device (
                     Arria® series, 
                     Cyclone®
                  series, and 
                     Stratix® series) family devices you can assign
                  nodes and entities to VREF groups using the Assignment
                  Editor, Pin Planner, and Chip Planner.