There was a lot of options explained using complex computer terminology that most cannot understand - so I deleted it. Here is the stuff I deleted.
Here is the same thing specified in confusing computerspeak:
Specifies in which of the three color spaces (RGB, ABI, HCI) the delta controls will work. Both ABI and HCI are separated luma/chroma. HCI is a cylindrical "Hue Chromacity Intensity" space, and ABI is a cartesian version thereof. Both suffer from chroma subsampling, and will give less sharp results than RGB. Note:HCI is slow, because it has to calculate atan2() and hypot() for each pixel.
Subspace Shape options are: Box, Ellipsoid, Diamond
Determines the shape of the color subspace. Options are: box, ellipsoid or diamond. Box is the biggest of them (by volume) and diamond the smallest. Imagine an octahedron inscribed inside an ellipsoid, which is in turn inscribed in a box. The tips of the diamond touch the ellipsoid, and the box, at the center of the sides of the box.
Operation options are: Write On Clear, Max, Min, Add and Subtract. This defines how to deal with an existing alpha channel in the clip. See Alpha Shapes for the meanings of these operations.
The "min", "max", "add" and "subtract" options allow cascading of select0r plugins (or combination with other alpha-writting plugins). These options combine the current selection with the pre-existing alpha of the source material. This way complex selections can be built.
Delta XXXX: These three parameters determine the size of the color subspace along each axis. Bigger value means bigger tolerance on that axis.
For example, setting a high I delta (in ABI and HCI), will allow the selection of a specific color in both light and shadows, but will also discard most of the high-bandwidth luma signal, making the selection less spatially accurate.
Slope: When "Edge mode" is set to "Slope", this parameter sets the gradualness of the transition between opaque and transparent