is blank / is not blank will allow you to use individual mapping methods for individual products or product groups.
You can assign some values to products depending on whether a given value is blank or not.
You can use this option for a Variation Theme attribute to Color or Size. if the source's item specifications Color is blank the Size value will be set and vice versa.
It is always recommended to have a Fallback rule for products that are not matched by the condition, just to make sure that there are no empty values in your output feed.