Usually I don't bother with them lol, but it says in the book,
"Those in which part of the Square is left vacant, the Letters being
arranged in the form of what is called in Geometry, a Gnomon."
A Gnomon magic square is 3x3 array where the elements in the 2 x 2 corners have the same sum. I think I remember Crowley saying in 777 in regards to the Sepher Sephiroth that a blank "-" indicates a number or letter divisible by 3, if that helps.
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