It is possible for the teacher to specify that some activities are required to be completed to achieve course completion and that others are not. However, there is no indication to the student of which is which other than going into the details page via the "Course completion status" block (if the teacher has remembered to include that even) and map back the list their to the main course page.

It would seem desirable that the status of each activity in respect of course completion should be immediately obvious - preferably on the course page (icon/colour maybe).