In > Core > Users/Access > Profile > Manage Roles, there are two roles that involve Grades in the On Suite, Grade Book Manager and Grading Manager. These roles do completely different things in the system
The Grade Book Manager role allows user to manage all Grade Book through onCampus. This role might be given to a Department Head, so they can take a look at all the Grade Books for their Department or an Academic Dean so they can look at all Grade Books for their School Level. There is not a way to limit this role by Department, but you can limit it by School Level.
The Grading Manager role allows users to use the Grading tasks in onRecord. This includes but is not limited to Report Cards, GPAs, Grade Plans, etc. This role would be given to the person in charge of creating your school's Report Cards or is involved in the Grading review process. There is also a way to limit this role by School Level and Group Type.