The part of the device silicon to which pins are bonded. The location of the pads determines the function of the pins connected to them, such as JTAG programming, I/O, ground, power, and so on. However, only pads connected to I/O pins can be assigned.
To avoid interference between VREFs pins, you must assign output signals with at least two intervening pads between them and the VREF pin, or just one intervening pad if the intervening pad is a VCC or GND pad.