I just recent questioned Leaf in Hong Kong on exposure for my A65, following was their reply. I think it clarifies a bit;
1. "+2" is the white point. Pick a point of white spot and the reading should be "+2". Then your exposure is at the normal range.
2. In digital back, we get the largest dynamic range already. In theory, the overexposure is "+2.5" and underexposure is "-4.0". All the data out of this range are hardly to retrieve. If you still need to retrieve the "shadow details", you can push up and get up those data in the range of "-5.0" or even "-6.0". But those information for sure would be very noisy. So, when you are shooting, you can use the "exposure alarm" to check would those information falling into your target objects or not.
3. "12 f-stops" DR is counting from "+3.0" to "-9.0". This is the specification from CCD sensor and the A/D converter. By theory, we set "+2" as the highlight point, "+0" as the mid-point and "-4" as the shadow point. From the histogram, you can see the curve setting is starting from the "+3" to the far end. If you don't adjust the toning curve, the recommended range is from +2.5 to -4.0. But, you can adjust the curve freely and retrieve the information from those out of normal range area. This is the power of the "12 f-stops" DR.
In contrast to the DR in traditional film, those out of normal range area are always missing details (physically washed out in film processing). So, the DR of film is only up to 5 f-stop.
Regards
Anders