Coin control lets you choose which of the individual amounts in your Bitcoin balance your wallet may spend for a payment. In the BitBoxApp, your selection limits which funds the app is allowed to use.

You do not need coin control for every payment. Automatic selection is suitable for ordinary sending when you have no particular source requirement. Coin control becomes useful when you want to keep certain funds from being spent together, use a specific output, or deliberately combine smaller outputs. Its privacy benefit depends on the choices you make; turning it on does not make transactions anonymous.

How UTXOs and change work

The individual amounts that make up your Bitcoin balance are called unspent transaction outputs (UTXOs). Each is an amount from an earlier transaction that is still available to spend. One address can have several UTXOs, so choosing an address and choosing a particular output are different things.

When you spend a UTXO, it becomes an input to a new transaction and is spent in full. The new transaction pays the recipient, pays the network fee, and usually returns any remaining value to your wallet as a new change output.

For example, suppose a payment spends one UTXO worth 100,000 satoshis. If you send 60,000 satoshis and the fee is 1,000 satoshis, the transaction creates a 39,000-satoshi change output for your wallet. The original 100,000-satoshi output is now spent; you receive a new output for the remainder. A satoshi is one hundred-millionth of a bitcoin.

The BitBoxApp handles change automatically. You do not choose a change address manually. If the remainder would be too small to create a usable change output, the app can add it to the fee instead, so check the actual fee before confirming. For more background, see what a UTXO is.

When coin control is useful

Avoid spending different sources together

Spending several UTXOs in one transaction creates a public connection between those inputs. For example, combining an exchange withdrawal with a payment received privately can make their histories easier to associate. Coin control lets you limit a payment to outputs you are willing to spend together.

This can avoid an unnecessary connection, but it does not erase earlier links or guarantee privacy. Change and later payments can create further connections. Use a new receiving address for each payment where possible; coin control does not undo the history exposed by address reuse.

Spend from a required output

A service may require a small payment from a particular Bitcoin address or output. Coin control lets you restrict the available inputs to the required source. If an address has several UTXOs, the service may need to specify which one it accepts.

If a service explicitly requests a Satoshi test, follow the dedicated Satoshi-test guide. Such a payment demonstrates the ability to spend the output used for the test; it does not establish your identity, legal ownership, or control of every address in your wallet. Acceptance depends on the service’s requirements.

Consolidate smaller outputs intentionally

Frequent Bitcoin receipts can leave you with many small UTXOs. Consolidation spends several of them in a payment back to your own wallet, creating a larger output.

Bitcoin transaction fees depend on the amount of transaction data and the fee rate, rather than simply on how much bitcoin you send. More inputs generally require more data. Having fewer inputs to spend later can therefore reduce the size and cost of a future transaction, especially if you consolidate at a lower fee rate than that future payment would use.

Consolidation itself costs a network fee and links the inputs you combine. It is a trade-off, not guaranteed savings or a routine requirement. Consider both the fee and which histories you are willing to connect before proceeding. For a fuller explanation, see UTXO consolidation and transaction fees.

Identify your funds with transaction notes

To choose which funds to spend, you need to recognize them. Transaction notes can help you remember where funds came from or why you received them, rather than relying on the amount alone. See managing wallet labels for the available label-management options.

Notes are an aid to your decision; they do not keep funds permanently separate. An output actually spent is replaced by new outputs, so consider the change and later selections as well.

Use coin control in your wallet

For the settings, output fields, and sending steps, continue with how to use coin control in the BitBoxApp. With a fixed payment amount, the app can use a subset of the outputs you selected; Send selected coins spends the selected set, minus the fee.

If you use Electrum with your BitBox02, follow the separate Electrum coin-control guide, which describes that wallet’s controls.