What is the method to make a field, like a featured flag, exclusive to only one element in the field lineup?

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One method to make a field exclusive to only one element in the field lineup is to use a validation rule. The validation rule can check if the field is already set for any other element in the lineup, and if so, prevent the user from selecting it for the current element. Another method is to use a workflow rule or a trigger to automatically unset the field for other elements in the lineup when it is set for a new element. This ensures that only one element has the field set at any given time.