Somehow, I've never even thought of using those "Payment" function buttons!
It looks like "Payment" with the $ sign will apply the payment as Ronald describes, without requiring the choice of a transaction to apply it to.
And the "Payment" with the + sign will put the payment in the "unused payment" window, requiring us to choose a transaction to assign it to.
They also give quite different receipts. Ronald, could you maybe explain the specific circumstances that each of these would be useful for?
Thanks!