The SBVR^[1] definition for rule is deeply embedded in formal logic, which deals with propositions.  In modal logic, propositions can claim different modes.  For business rules the two relevant modes are alethic and deontic.

• Alethic rules are true 'by definition'.  As such they cannot be violated.  They are about how concepts, knowledge, or information are defined or structured.


• Deontic rules are rules that can be violated.  They are rules about behavior, not concepts, knowledge, or information.  Deontic rules are really about people, what they must and must not do, even if their activity (and the rules) are automated.

Both kinds of rules are important, of course, but deontic rules — people rules — are especially so since, ultimately, businesses are about the activity of people.  This situation is very different than in the semantic web, for example, where it's all about only knowledge.

Under modal logic, every rule must therefore 'claim' one of two modes.  (In practice, the 'claim' arises naturally from the syntax of a rule statement or as a meta-property.)

• Alethic implications (rules) are established by 'claiming' necessity.  Things are necessarily true.  A concept is what it is; says what it says.  That's just the way things are.


• Deontic implications (rules) are established by 'claiming' obligation.  Behavior is 'obliged' to follow the rule.  But, of course, people don't always follow the rules so there can be violations.  Major difference.

So a rule 'claims' either necessity or obligation, which establishes what kind of rule it is.  Therefore the SBVR definitions for the two kinds of rule are:

• Definitional rule:  rule that is a claim of necessity


• Behavioral rule:  business rule that is a claim of obligation

Why doesn't the definition for definitional rule say "business rule" like the one for behavioral rule?  Because some definitional rules are not 'under business jurisdiction' (in other words, business has no choice about them).  Examples include the 'law' of gravity and all the rules of mathematics.  Those rules are simply universally true.

I recommend the following pragmatic definitions for business analysts.

• Definitional rule:   a rule that indicates something is necessarily true (or untrue); a rule that is intended as a definitional criterion for concepts, knowledge, or information


• Behavioral rule:  a business rule that places an obligation (or prohibition) on conduct, action, practice, or procedure; a business rule whose purpose is to shape (govern) day-to-day business activity

Synonyms:  Early in the development of SBVR, I introduced the terms structural rule and operative rule for the two kinds of rules, respectively.  That was before the full implications of modal logic became clear.  Since then the terms definitional rule and behavioral rule have become preferred, even in all internal SBVR discussion.  The synonyms, however, remain valid.

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References

[1]  For more information about SBVR see the SBVR Insider section on www.BRCommunity.com.  

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