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Image Removed  What if an attribute value is set by a default value as well as a business rule? What if both a business rule and a decision table exist for the same attribute? How is the sequence affected when the attribute default is a reference to a decision table? etc.

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The inference engine first checks whether an attribute is user-set.
  - If so, all rules and defaults are ignored.

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If not, the inference engine tries to find a rule amongst all decision tables, business rules, external rules and data rules that can set the attribute. Therefore the inference engine evaluates the condition of these rules.
  - If more than rule is found that can actually set the attribute, an error is displayed.
  - If exactly one rule is found that can set the attribute, that particular rule is fired.
  - If no rule is found that can set the attribute, the default value (default constant, default expression, default decision table or default external rule) is used to set the attribute.

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Image Added  It is possible to set an attribute to unknown in a decision table. This is sometimes referred to as clearing the attribute. In a lot cases, the service AQ_Instance_Update can and will be used to achieve this, but if the condition in which to clear the attribute is modeled by logic, a decision table may be the best means to clear the attribute. See the example below.