Extended notes are XML documents that link a note text to one or more objects (references, keywords, author names, periodicals) in your reference database. You work with these data basically in the same way as with risx reference data. nxml-mode povides real-time validation, and the Add Notes and Update Notes commands also refuse to run if the buffer is not valid.
Retrieving notes also works in a similar fashion as retrieving references, but with an additional quirk: as notes are linked to database objects, you can use the link target as a criterion in notes queries. Wherever appropriate, minibuffer completion and history are also available. The relevant functions are located in the Get Notes and Get Notes on Region submenus.