Maurice
This would be a single motor to move the camera to change focus and then software for stacking the images. This would be for macro of 2x or more. My experimenting has be with a diy slider, CA5 (intervalometer), a 40D and a MPE 65 mm lens. Did try the stacker module in the CA5 software V5.5, with out success, however, I must point out that I did not spend a lot of time working on iit. With the CA6 and a dedicated focus motorised focus rail should, hopefully, make life a little bit easier. The CA5 intervalometer method worked pretty well, about 80 % success. Helicon Focus is my software.
Dan: I have found that the distances covered by a macro stack does not make much difference to the focus, whether you use camera move or the the lens focus. Ideally, it would be great if one can use the lens to focus, however,the MPE 65 mm does not give me that choice.
Wilfred