Let's say the channel you are advertising on, such as Google Shopping, wants you to provide the direct product link URL that would show the customer the exact product variant that is being advertised. In order to get the true URL for a variant, you will need to create the following custom rule in the desired feed. Note, you must choose the [F] and [P] field names from the drop down lists, not copy and paste them.
TO [P.][absolute_product_url] + ?variant= + [P.][id]
WHEN [P.][absolute_product_url] is not empty