When an assignment is created in OnCourse Classroom, you have the option of adding it to OnCourse Grade Book. Once the link is created, the assignment title is visible in both applications. Then, after the assignment is graded, the grade will be visible in both applications as well. Later, when an assignment comment is created in Classroom, it will automatically sync with Grade Book. The same is true if a comment is created in Grade Book, it will be visible in Classroom.
Note: Only public comments will sync. Internal comments will not.