In order to export your client's transactions as CSV, use the export CSV feature on the bookkeeping tab. Bear in mind that export will use whatever filters you apply to the page.
To export all transactions, make sure all dates are selected and that no filters are applied. To get a specific date range, use the date picker. You can use quick selectors for the month or tax year, or select a custom range.
To export specific types of transactions, you can use the category review filters on the left, or a search filter.
Once you export your data you see the following fields, including the original transaction name before Coconut tidied it up. It includes separate columns for credits and debits as well as a column for amount that includes both directions.
Apart from the category you can also see the relevant SA103S category that you can use for preparing self assessments figures.