A remittance advice tells you a payment has been sent and what it relates to — essential when one bank transfer covers several invoices. Larger companies send them automatically from their accounts system.
If a client says "we paid" but you cannot see it, ask for the remittance advice: it shows the date, amount and the bank account the money went to.
Example
"Payment of £3,450 sent 14 Oct covering invoices INV-0012, INV-0015 and INV-0016."