When you add a payment to an invoice or mark it as paid, the invoice gets one of the following payment status reasons: 

The invoice payment status reason ‘NoPaymentInfo’ means that the paid amount is less than or equal to zero and the PO# field is empty. 

The invoice payment status reason ‘PaymentInfoReceived’ means that the paid amount is greater than zero or the PO# is added. 

The invoice payment status reason ‘NotPaid’ means that the paid amount is less than or equal to zero and less than the invoice amount. 

The invoice payment status reason ‘Paid’ means that the paid amount is greater than zero and is equal to or greater than the invoice amount.