For example, the line “1    3    21   2.57” indicates that reflection nos. 1, 3 and 21 participate in a Σ₂ relationship. The “E3-VALUE” is equal to (1/N^½)E₁E₂E₃ (N is the number of non-H atoms in the cell) and is a measure of the “strength” of the particular Σ₂ relationship (the probability that the &Sigma₂ statement is true is proportional to the sizes of the normalized structure factors involved). It is important that those three origin-defining reflections be three of the “strongest” reflections (i.e., that they have a large number of high probability &Sigma₂ relationships with other reflections). This is certainly true of reflection no. 1; it is less clear why the program picked no. 4 (even though reflection no. 4 doesn't have as many relationships as some of the other top reflections, maybe, on the next level, the reflections it's related to have a large number of good relationships?)