A proof of sources of funds (POSOF) is any (collection of) document(s) that explains where the funds used for a fiat deposit originated and where the crypto assets used for a fiat withdrawal originated.
Kraken may request a proof of source of funds for US-based clients depositing and withdrawing USD using SynapsePay
Kraken reserves the right to ask any client using any funding methods for a POSOF, but such requests tend to be more frequent when a client’s lifetime deposits (withdrawals) exceeds $15,000.
Any POSOF document submitted needs to cover all deposits (withdrawals) via that particular funding method. If a client has made deposits (withdrawals) earlier they should make sure that the POSOF documentation covers any previous deposits (withdrawals) as well as the most recent one.