It's possible to make parts mandatory within an e-learning or exam. This can be done by adding conditions or completion requirements.
This works differently for a playlist. When you add activities to a playlist you can also choose to make some activities mandatory. This can be done by first adding learning activities to the playlist. Next up, you press the sprocket and you can indicate which activities should become mandatory.
You can also choose to add conditions to a playlist. You then determine a fixed order. This option can also be found by clicking on the sprocket.