sculptie is using the color channels for xyz coords so...

what does each channel unit refer to ? how much unit is 1 r value?
if i wanted to sit here making a image in paint.net or something what would i expect if i made the the first pixel on the top left r=100 g=100 b=100?

1 r value is 1/256 of the X scale. So if the sculptie was scaled to 2.56m then 1 r is 1cm. If it was scaled to 0.256m then 1r is 1mm.

Red is X, Green is Y and Blue is Z, Each axis can be scaled seperately and you should treat the entire square of the sculptie map as though it was a sheet of rubber wrapped around the object. So you need to use gradients rather than just painting any old positions next to each other.

The most amusing thing to try to grasp about 'sculpties' is how color (on the sculpt-map) determines location while location (on the sculpt-map), indirectly, determines color.

But anyways, good question; the answer has been ever-so-slightly tweaked at least once. Like any prim, a sculpted prim has 3 dimensions to it – its boundaries – which can each be stretched out individually (or simultaneously to maintain proportions) in-world. In your example, a darkish gray <100,100,100> pixel in the corner of the sculpt map – well actually, ANYwhere on the map; it doesn't matter – will be placed as a vertex at 100/255 (this used to be /256) of the way in each dimension from one side of the boundaries to the other.

Regarding the significance of a pixel's PLACEment on the color map, all there is to understand is that
(1) pixels next to each other become vertices that tie together to create polygons,
(2) pixels on the edges tie together differently depending on settings, with the default being 'sphere' mapping. In such mapping, they tie together exactly like a flat map of the earth, with poles on top and bottom while the
(3) if you arrange it backwards, the sculpty will render inside-out. (expect this to look very weird.)
(4) remember what I said about determining color? When you apply a texture atop your sculpty in-world, by default, the vertices from one part of your sculpt map end up with the colors of the same relative position on the texture (the left side to the left, the right to the right, and so on). This of course changes once you change the texture offset/scale/rotation.

This is discussed in detail in the Wiki.