It can’t be opaque channel. Why? because opaqueness formula affects the entire element. Alpha only affects the color. An alpha channel is a linear gradient not just 0 and 1. So in alpha compositing, which later was added as a color channel called the alpha channel in some image formats, the alpha controls the amount of mix between 2 images. The mathematical formula uses the Greek alpha symbol(α) as the linear value. Thus how it got it’s name. The other 2 variables where A and B which is why you see it listed as image/video A to image/video B in most video editors. Sorry I don’t remember the formula. In this case, you are mixing the texture’s color (A) to the color behind it(B). This is similar to how the factor(Fac) on a mixRGB/Mix color node set to mix. The top color is the background(B) and the bottom color is the color being mixed(A). The Fac would be the alpha. Fac of 0 would be fully the background color. Fac 0.5 would be a 50% mix of the bottom color with the top color. Fac of 1 would 100% the bottom color. Note: Since this has more to do with video editing/color grading I don’t think CGC covers this, but just for your information. Opacity compositing using A over B usually gives the same results as alpha compositing. With Opacity compositing you also have A in B, A atop B, and A out B which all give you different results and honestly I’m still try to figure out.
