Quick question about away rotations...

How do schools find out where you did your aways? Is that part of ERAS, or is it only by who writes your letters or by what you tell them on the interviews?

Basically I'm aiming to break into a new geographical area, but the way my schedule looks I may not be able to do aways there until the Sept/Oct timeframe. I realize this make getting a letter from those aways submitted in time for other schools to see it unlikely. Are letters from the aways crucial to getting interviews in that area?

Thanks for the help...