This function can only be used on fields within a node having value Loop in node attribute Iteration set and only makes sense for option normal data mapper.

Aborts the value assignment (mapping process/function execution) for this field if it already had a value assigned in a preceding iteration (by the mapping or a function), otherwise, false is returned. Note: If an empty string ("") has been assigned to the field, it is considered to be an assigned value. The Empty Flag has no relevance for this function.