To allow a feedback plugin to add elements to an assignment grading form the function 'get_form_elements' is called.

This is passed 3 parameters - grade (which is false, if the submission has not yet been graded), mform (the form to add elements to) and data.

If the submission already has a grade, then the 'grade' parameter includes the userid for the assignment that is currently being edited, which can then be used in whatever way required (including looking up the submissionid).

If the submission does not yet have a grade, then there appears to be no sensible way to retrieve the userid for the current submission (or the submissionid, for the submission).

For many feedback plugins, this may not be a problem (as they do not need the userid until the form is saved, by which point the grade will have been created), for my feedback plugin I need to know the userid, before I can start the grading process.