Since a lot of people are curious about the purpose of the ATN* pin on the Arduino (Genuino) 101, I did some research and found the answer in the Arduino.cc forum, so here it is:
Hi! The ATN pin is an experimental feature we're introducing with the Arduino Zero. Many shields and expansion modules use an SPI communication protocol and this always requires electing a pin to act as "chip select" effectively making one extra pin unavailable. On processors that have more than 28 pins there are usually extra unallocated pins that can be used. We decided to experiment and allocate a pin to act as the default chip select for the first shield/module on the bus.
By cmaglie, administrator of the Arduino.cc's forum.
*This pin also exist on the Arduino (Genuino) Zero board.
Link to the forum topic - here.