What is a proration
Proration can happen if you set a future cancellation date on a subscription and it is before the final billing date.
Example of a proration
A client's subscription renews on the 1st of every month
On December 6th I set a cancellation of Jan 15th then invoice would prorate for Jan 1st- 15th of January. This is because the client would be charged on the 1st of the month (as normal), but not for the entire billing cycle.