Explanation/Reference:
If transactions cannot be matched or transaction information is not available, Receivables uses the AutoCash rule set defined for the customer profile either at the customer site or customer level to apply the receipt. If the customer does not have an AutoCash rule set assigned to a profile, Receivables uses the AutoCash rule set assigned to system options and the number of discount grace days defined in the customer site or customer profile to apply the receipt.
If none of the rules in the AutoCash rule set apply, Receivables places the remaining amount either unapplied or on-account, depending on the setting of the Remaining Remittance Amount option on the AutoCash rule set.
References: http://docs.oracle.com/cd/E25054_01/fusionapps.1111/e20375/F569967AN5E21B.htm