The NOT (
!) operator is used to reverse a logical value. That is, if a condition returns a TRUE, using NOT will change its value to FALSE, and vice versa.
|The condition to negate
If you have a form with an optional text field with the code
REGISTRATION_NUMBER, you could add a field "Registration authority" with the relevance rule:
In this case, the registration authority field will only be displayed if a registration number is provided.