I see that what is being done there is using the transmap as a mixer to single-channel color and transparency channels. I'm assuming that the advantage here over using the transmap in a mixer in the color channel of a multichannel material is that Carrara would render the "source 2" color over the transparent parts? So by using this single channel system, Carrara then applies a value of "nothing" to the negative areas on the color map? I will give this a try as soon as the current render is over (says the clock, in about four hours!)