@CrossRoads1. For Arduino UNO System, the underlying board is the Arduino UNO Kit. Am I correct?2. For shield, the peripheral board that will be plugged-in on the top of Arduino UNO through one or more of the 4 female connectors of the Arduino UNO. Am I correct?3. I have physically checked that the IOREF-pin of Arduino UNO is solidly connected with 5V.4. The operating voltage of ATmega328 of Arduino UNO Kit is 5V. So, the logic levels of the IO pins of the Arduino UNO is expected to be within (0 to 5V) depending on loading conditions of the IO pins.5. In Arduino UNO Kit, the 3.3V is a seperate point which is solidly connected with the output of an on-board 3.3V regulator (IC6, Reference Design). The Arduino UNO has not used it.5. So, the meaning of your following sentence ofyour post is not clear. Would appreciate clarification with example: It is to let a shield know what voltage levels to expect on the IO pins from the underlying Arduino.