I go to Accounting > Customer Payments and click create. I enter the customer name. I see a list of the customer's invoices and refunds. I select an invoice or refund and then click to fill the Reconcile check box. Sometimes it sticks, sometimes it disappears immediately. There seems to be a rule that allows/disallows the checkmark. What is it?

Also, sometimes, it seems like a refund will get it's reconcile box checked on it's own even though I never selected that refund. There again, it seems like there are rules in effect that auto-check a refund. If these rules existe, where are they listed?