I think when you add a condition to a rule, it becomes an AND. When you add two or more new rules for the same conditions they become an OR. So in general if you create three rules, each with two conditions and all with the same resulting action, you could view it as IF (A+B) OR (C+D) OR (E+F) THEN ACTION. A,B,C&D are not necessarily unique.
Note that in the example above, the three rules are evaluated individually, not as one complex rule. So it's possible to generate multiple identical actions as a result, which may lead to unexpected results, depending on their nature.
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