is longer than / is shorter than will allow you to use individual mapping methods for individual products or product groups.
You can assign some values to products depending on the number of characters.
You can use this option for the product description attribute. if your source's item description is longer than 150 characters but the channel only requires a maximum of 130 characters. You have an option to set the item features as the listing description. Fortunately, the source's listing description will be set if it is shorter than 130 characters.
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.