The answer depends on the router.
If you have a nice, quality router with updated firmware, "Auto" will do a quick scan for what channels have the least interference, and pick from the best option.
If you have a cheap router, it usually defaults to channel 6 or 8 if you choose Auto. Since that's what most people have, you end up with a whole neighborhood living on the same channel and wondering why their internet connection sucks, even though the ISP keeps telling them they're getting full speed when they test from the Modem.
I recommend getting an app for your cell phone that will tell you how many different wifi broadcast signals it detects per channel. I use one on my android device that shows me a graph of each device's strength from my location, and the Network Identifier for the device. Using this, I was able to discover that my neighbor's irritating WiFi speakers in his back yard were single-handedly tanking most of the connections for the surrounding houses on channel 6. I manually set my 2.4ghz bandwidth connection around this until I moved up to a 5ghz router, which put me well outside of locally used frequencies.