In my wallet gui v0.14.0.0, each outgoing transaction has a P button, the content when clicked is "Payment Proof", second line is "OutProofV1...(a hash string)" How do I use this payment proof ? And I notice that when the transaction has just been made, the hash string is so much longer. Why is that ?
Payment proofs are detailed nicely on the Monero site here: https://www.getmonero.org/resources/user-guides/prove-payment.html
They are used in the case of a dispute so you, as the spender, can prove to the receiver you sent funds to them.