In Propel today, we have the ability to set a recurring charge at the option set level.
For example, I want to sell a t-shirt. I want to charge a recurring charge based on the size of the t-shirt size. Today, I believe this can only be done as a flat rate recurring charge, not specific to what options the customer has selected.
For a real world example - a customer is going to order a server - the recurring charge would be different as the customer selects RAM, CPU and Storage options.