auto cash rule question

When a payment is auto-applying, what is the secondary criteria after due date? 

If you have several invoices with the same due date what invoice is applied first?

The reason I ask – when we have a customer that receives discounts for paying in advance of 15 days. 

If they have a 30 day invoice that is paid on day 16, it does not receive a discount, even though it is paid early (discounts end on day 14). 

So, if a payment arrives today, and will cover 5 invoices.  The next due date has 10 open invoices, 5 are 10 days old (would receive discounts), and 5 are 25 days old (no discount), what would Oracle pay first?