On Mon, 16 Mar 2015 15:19:36 -0700 (PDT), sid
wrote:

My attic is divided by a piece of foil backed insulation board. its only about 1/8" thick, but I need to cut a piece out to get to the other side of the attic to hang a ceiling fan. Don't understand why it is there. Either side of the attic is open to the vents in the eve, don't what purpose it would serve.
Maybe to stop breeze from coming in the eve on one side of the house through the attic and out the other removing any heat trapped there in the winter ?

Any help is appreciated.

Fire stop is my first thought. Doesn't make sense being 1/8 " thick.
I'd expect 5/8"? Have you asked your local building code office?