UTXO

Bitcoin doesn't keep account balances. A wallet holds unspent transaction outputs (UTXOs): individual coins of whatever size they were received in. To pay, a wallet spends one or more whole UTXOs and creates new outputs: one to the recipient, and usually one back to itself, the change.

Consequences for a business:

• Fees depend on size in bytes, and a payment that spends many small coins is larger, so it costs more.
• Coins can be in flight. Until a payment confirms, its change isn't confirmed either, so spending it right away chains payments together.
• The fee comes from the coins being spent, so no separate gas is needed.

On Valtu

Bitcoin wallets use Native SegWit addresses. While a payout is unconfirmed, its coins are reserved: the next payout waits for confirmed change instead of spending the same coins, so rapid payouts don't conflict. Every Bitcoin payout records its coins and which output is change, which is what allows replace-by-fee. Sweeps also pay their fee from the coins they move.