The faq/how-to on terrain editing has a slightly different listing of the channels than photoshop. In PS CS2 there are actually 14 channels because it adds an RGB channel at the top. So my question is... which of the Alpha channels actually allows you to create parcels?
The RGB thing at the top isn't a channel. It's an aggregate view of the first three channels combined. Photoshop assumes the first three channels are for Red, Green, and Blue, respectively, but the Second Life RAW format follows a completely different scheme.
Note that the chart in the Private Estate Guide TRIES to tell you the correct channel names as you'd see them in Photoshop, but at least in Photoshop 6.0 (which I use), their labels are incorrect beginning with channel 4. They ARE listed in the correct order though in the estate guide's table, even if they're labeled wrong. I've successfully used the estate guide's channel descriptions to develop several island designs.