On the contrary, I’m glad to explain how PhF works! However, sometimes I might not be very quick in answering 
The short answer: nothing! The longer one: this is a “buffer” layer, which is a sort of no-op tool. It does nothing, and it is just used to “give a name” to a specific point in the layer stack. In this specific case, I only used it to visualise the output of the mask while editing the other layers in the mask.
Yes, the layer clone in this case take the data from another mask layer… this is perfectly legitimate.
The difference between cloning the blended or not-blended version of a layer is that, in the blended case, one copies the result of the blending step. If for example a layer is blended with 50% opacity, cloning the blended version will give you the result of the 50% mix, while cloning the non-blended version will give you the layer data before being mixed…
The “plot input” is again a buffer layer, it is only there to “give a name” to the input data of the group.
The “split details” does the wavelet decomposition. On top of it I put a copy of the input data of the group, in the “split input clone”. Above the input copy, I put a copy of two of the detail scales, blended in “grain merge” mode… this effectively adds high-frequency details to the original image, thus obtaining a sharpening effect. This is basically a version of the wavelet sharpening technique.
The “split details” filter is a quite special one, in that it produces more than one output image. The individual output images can be accessed with clone layers, using the “sub-image” parameter, like this:
I hope this answers at lest few of your questions…