Use of Whole forum grading is meant to be an alternative to grading via Forum ratings. This feature is new as of Moodle 3.8 and allows for viewing all the interactions of student within a given forum in one place where you can provide.
At this time feedback is not supported within this grading interface but can be done by choosing to reply privately to a post while normally viewing a forum.
NOTE: This functionality is NOT available to Open Forums.
Enter the forum in question
Click the Grade users button
Each page will list a student and all of their interactions will appear in the left portion of the page.
If additional context is needed to grade these posts you can click the associated View parent post link or View discussion button.
Enter the number of points earned by the student in the Grades field to the right of the page or click the relevant cells of the Rubric if using Advanced Grading.
OPTIONAL: If you’d like your student to receive an email notification that this forum has been graded, under Notifications click the Yes, send notification to student item.
Click the Save button at the top-right corner of the page.
Navigate to the next student by using Search users (magnifying lens icon) or the change user arrows to navigate between enrolled students.
Repeat as necessary.