In our original paper, we defined 'business rule' from the conceptual, information system (I/S) perspective. We readily acknowledged this narrow focus, which was consciously chosen to let us develop consensus on a set of core concepts. At the same time, we identified a broader scope for future investigation. Our current work now focuses on one part of that broader scope ~ on the business perspective of business rules.
Why multiple definitions?
From the I/S perspective, a business rule pertains to the facts of the system that are recorded as data and to the constraints on changes to the values of those facts. The business perspective of business rules involves the behavior of people in the business (human activity) system. Because these perspectives are distinct, we have a definition of 'business rule' for each of these perspectives.