An assignment applies as a separate unit inside a chapter. For that reason assignments count in to your course progress. You can recognise the assignment through the pencil icon in the chapter overview above the main resources.
An assignment can be built of single choice, multiple choice or free text questions. Furthermore it can be a learning journal or a peer to peer assignment.
Please keep in mind that there is a difference between assignments and exercises. While assignments count into your progress and are therefore obligatory, exercises are voluntary. Exercises should help you to implement your knowledge. For that reason exercises are not related to your progress.
In general, you are free to choose the degree to which you want to engage in your course. So you don't have to do the assignments if you don't want to do them. However, if you wish to receive a certificate upon course completion, you will need to comply to the instructor’s requirements, which might be to do the particular assignments.