The sensor in the 2600 includes what Sony calls the High Conversion Gain (HCG) mode.
Since the ZWO camera protocol does not include an HCG flag, what ZWO apparently decided to do is to turn HCG on in the ASI2600 when the gain is 100 (10 dB) or greater. When the gain is under 100, ZWO sets the camera to Low Conversion Gain (LCG) mode.
So, a gain of 100 is essentially the lowest gain possible when you want HCG to be turned on. HCG is very helpful to reduce read noise, while not reducing the dynamic range of the camera by very much.
At the same time, gains of over 100 is not useful since the read noise at a gain of 100 is already so low that increasing the gain will not practically change the read noise. At the same time, increasing the gain will only reduce the dynamic range. I.e,, increasing the gain to greater than 100 has no practical advantage, but has real-world disadvantages.
I think this is the reason why ZWO supports only two gain values in ASIAIR (0 -- i.e., unity gain, and 100 -- 10 dB gain). You use a gain of 0 only in very special cases when you really need an additional 1/3 f-stop of dynamic range.
The dynamic range of the 2600 at gain of 100 is already extremely large. I have managed to take M31 with just a single exposure time, so that the innermost core barely saturates, while the dust lanes still show through nicely. With lower dynamic range cameras, you would need to stack subframes with different exposure times.
Take a look at charts in your camera's Manual under "Camera Performance." It will be quite obvious. Look at what happens to the Dynamic Range and the Read Noise when you cross the 10 dB gain (gain=100) threshold.
Chen
