They are different ways to measure students’ performance. 
By default, they show how students are doing with course materials (Practice Extra, Unit Progress Tests, etc).

You can also add other things to grade, like behaviour, class projects, and attendance. 
These marks will be part of the final grade. You can also change how much each category counts using a percentage. 
For example, Practice Extra 60%, projects 30%, and attendance 10%.

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