hei amir,
check the file. a plane is defined by 3 points, therefore a quad does not have to be planar - Srf4Pt creates planar surfaces (100%) only for three points or 4 coplanar points.
you posted is a good illustration of this as 1 and 7 are the same and so is 5 and 6. They are just oriented differently.
You have from Left to Right: I, L, Left, N, O, O, I. All you are missing is T ( similar to L but with the protrusion in the middle), Right (opposite of left) and 3 (which is like Left but with the tower moved to the middle)
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Added by Danny Boyes at 5:46am on November 17, 2011
e image searches on the forum are useful as well:
site:grasshopper3d.com +Boolean +union +fails
https://www.google.com/search?q=site+grasshopper3d+com+Boolean+unio...
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Added by Nik Willmore at 12:13am on February 27, 2016