Deleting a transaction means it will still show in your wallet, but it will not be included in your calculations or billable transactions. If you sync the wallet, it will not reimport this transaction.

Permanently deleting a transaction will remove it entirely from your wallet. Once transactions are permanently deleted, they will be reimported if you sync the wallet from the beginning if they are API-imported transactions (see article here - How to resync your wallet from the beginning).

If you permanently delete transactions imported via a CSV file, the CSV file would need to be imported again to restore these.

Perm. Delete is useful if you have imported data that is incorrect, for example, from importing incorrect CSVs or if there has been a problem with the API, and you want to import the data as if it were the first time.

To delete a specific transaction, click on it to expand and then click the 'bin' icon:

This will delete the transaction - it will no longer be included in your calculations and will not count towards your billable transaction count.

To restore a deleted transaction, you can select it on the left-hand side and click Restore at the bottom of the screen:

To permanently delete a transaction that's already been deleted, you can select them on the left-hand side and click 'Perm Delete' at the bottom of the screen:

To show or hide your deleted transactions, go to the Transactions page, open the 'View' dropdown menu and toggle 'Show deleted':
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If you have a lot of transactions to delete, you can use the Bulk Edit/Delete feature as explained here - Bulk editing and deleting