The messaging system is public for followers of documents. If the customer is a follower, he can read all posts if he has access to the document. If he does not have access to the document, he will receive a notification by email on all posted/email message. By default he does not receive a notification for automatic messages like change of state but each follower can choose which kind of info he wants to follow by using subtypes.

The note field, available on the page of each document, can be used to write internal/private notes. If this field is used, only people who have full access to the document (hr manager for applicants, sales user for opportunity,...) have the right to see it.