Arduino/Genuino 101 5 volts pin
In Genuino 5V pin is a DC regulated pin. That means when you supply Power (9 to 12 V) to Vin or through the DC Jack, a 5V voltage regulator regulates to give constant output at 5 V. The purpose of that pin is to provide 5V output for other use- as a power supply to some sensor you are interfacing or anything.

However, you can power up the Genuino 101 via the 5V pin. But that is not advisable because in that case it will bypass the regulator present in the board and can damage the circuit if improperly regulated.