I'm trying to come to grips with this CCD feature and want someone that knows, to verify what I think
I understand this to be an area of storage within the sensor which is not connected to any pixels. Once the read of the sensor is done, the overscan area holds info on the read signal introduced by reading the sensor. Taking a mean of the overscan area allows you to remove this introduced signal from the image.
This differs to a bias signal which has a patern associated with it and is related to individual pixels. The Overscan level would therefore be classed as read noise and is uniformally removed from each pixel.
The overscan area should not respond to light.
Does that sound pretty close?