Signing off a transaction means confirming that it's okay and doesn't need fixing. When you're fixing issues in Clean-up Mode, you might find transactions that are fine as they are. In this case, you can sign them off to hide them from the report.
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For example, if a customer has a negative balance due to an advance payment, and you're okay with it, you can sign off the transaction, and it will vanish from the report. You'll see a tick mark under the Action column to sign off a transaction. Once you do that, it disappears from the report.

If you want to see signed-off transactions➝ check the option Show Signed Off Transactions ➝ it will display the signed-off transactions.

If you want to remove the signed-off status ➝ click on the same tick mark again, and it will remove the signed-off status.