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

NoPaymentInfo

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

PaymentInfoReceived

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

NotPaid

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

Paid

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.

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