Confidential information is any information that is provided to you by a client in confidence and as a result of your engagement with the client. This will often be all the information on a file. This is the safest approach to adopt if there is any doubt.
You should also be aware that at common law you will generally owe a duty of confidentiality to potential clients or ‘enquirers’, that is people with whom you have an initial discussion or consultation but who do not go on to instruct you, or who you refer to someone else, or for whom you decline to formally act (as is your right). This is so even though this may go beyond the strict terms of Rule 9 as in the Glossary of Terms 'engagement' means your appointment to provide legal services for a matter.