boxes in Figure4-14 give the chip pin number and port designations One is designated as IOREF (the nominal I/O voltage, may be either 3.3V or 5V depending on board type) and the other is not presently connected. The 5V/16MHz board runs at the same voltage and speed as the Arduino Uno. Arduino Uno Dimensions tell you how big the board is in terms of that a baseline (Uno, Leonardo, etc.) Table4-4 lists the extended baseline layout boards.