A payment records money out: to this supplier, on this date, from this bank or cash account, by this method. Like a receipt, its second job is allocation — naming the invoices the money settles — and the form suggests allocations against the supplier's open invoices. Where the supplier's payment terms offer an early-payment discount and you pay inside the window, the allocation records the discount taken alongside the amount paid: only the net leaves your bank, and the discount is accounted for openly rather than the invoice silently shrinking. Whatever is paid beyond the allocated invoices stays unapplied — credit with the supplier, visible on their record, against the next invoice.
Where the payment or the bank account speaks a different currency from the invoice, the difference against the invoice's frozen rate is recognised openly as an exchange gain or loss at settlement — the same law as receipts, in the other direction (1.11).