Account and Event Administrators can record 'Notes To Submitter' in the Decide Outcomes table ( click All Decisions in the Decisions section of the event dashboard).  If Committee Members have been given permission to record decisions they can also do this.  See also Recording a decision (Accepting/Rejecting/Withdrawing a submission) and Allow Committee Members to accept or reject submissions

Including Notes to Submitter in Emails

To include 'Notes To Submitter' in emails, click on the 'notes to submitter' button in the 'Merge Fields' section of the relevant email.  See Designing Emails