S

(Signed) indicates the numeric columns should be stored taking into account their signs (+ or -).

U

(Unsigned) indicates the numeric column should be stored without taking into account their signs. The numeric column is to be treated just like a character column.

A

(Absolute value) indicates that the numeric column should be stored by its absolute value. Note that this is not the same (and does not always produce the same order) as the U option.