What's that sensor on the back for?

[tbkfan]

I have a few cases for my Z1 already. The latest addition to the collection is one that I picked up in the Sony Store. It does not have a cutout that all the others have for what must be some kind of sensor behind the glass on the back of the phone to the right of the camera lens. Does anyone know what this assumed sensor is for and why some cases have a cutout for it and others do not? Everything seems to be working fine with this case that does not have the cutout, but I wonder if I am going to run into some issue in the near future.

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