Once you have created a tag, you'll be able to allocate it to any coded lines in invoices and bank transactions, using the Tag column.
NOTE: you can allocate a maximum of one tag to each coded line.
Edit a tag allocation
Go to the Completed tab.
Change the view to Coded lines.
Sort the view by either Code order, if you know what code you used, or by Tag order, if you know the tag you want to edit.
You can also click into a Transaction in the Completed screen and choose to view the Matched Invoice.
This will take you into the coded lines of the invoice where you can allocate or edit a Tag if required.