How does Oracle assign the check numbers to the payments in a PPR?

When invoices are selected for payment within one PPR file, how does Oracle assign the check numbers to the payments? Are the assigned based on alaphabetic order by supplier/payee name? Are they assigned based on invoice creation date/time? Are they assigned alpha/numerically based on invoice number? It looks like within one PPR, if we have different types of checks being paid, we are seeing them intermix. For us to control the sequential order they are printed in, we need to know how the check numbers are assigned.