Business Rules have several quirky behaviors that will be addressed in a future release. Please read and understand these points before writing your first business rule.
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The code above makes logical sense but it just won't work. In a nutshell this particularity of the implementation can be resumed explained this way:
The right hand side of an assignment expression resolves to the value of a control passed to a rule when it is invoked. The left hand side uses a reference to the control'.
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What causes an "Invalid signature detected. Data may have been tampered with" error?
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This error message usually displays in multi-step workflows when there is a rule on a subsequent step that executes at form load and updates values inside a signed section in a previous step. Check your rules if you encounter this error.