This dialog is shown as a result of importing or retrieving transactions.
All transactions available for update will be listed. To not update a particular transaction remove the check mark next to that transaction by clicking on the check box with your mouse. Click on it again to turn the check mark back on. You can also use the "Select All" and "Select None" buttons to check/un-check all transactions. Use the Ctrl and Shift keys to select multiple transactions either individually or in a range.
Edit or view more details for any transaction by double clicking on it, or right mouse clicking on it, and choosing "Transaction Details..." from the popup menu.
Sort the list by any column by clicking on the column header. Click it again to toggle between ascending/descending. You can re-order the columns by dragging/dropping a column header to the new desired location.
The price and value fields can optionally be labelled with the currency symbol based on the state of the option: "General Preferences... / Data Register Display / Display Price/Value Fields With Currency".
Choose the "Automatically Record Corresponding Entries in Default Cash Account" to have all the proceeds from selected transactions automatically be added/removed to/from the associated default cash account.