Every unit is divided into a main resource and additional resources. The important distinction ist that the main resource counts into to your progress. The additional materials help you to reflect and understand the main resource and to go deeper into the details. The additional resources can be a link document or to a website. It doesn't influences your overall progress in the course, but can help you to better understand the content and find some interesting additional information.
The main unit can be a text, a video or an assignment. Often there is a quiz related to the main resource. Please keep in mind that the solution of the quiz is needed for your progress. If the quiz or the exercise belongs to the additional resources it's up to you if you want to solve it. So if you are missing progress, maybe you left out one of the quizzes.
The additional resources consist of additional exercises which help you to understand and implement the things you've learned immediately. The work on the additional exercises are not influencing your progress.
The additional material can be texts, pictures or videos. They are related to the main resource and help you to learn more about the subject and to get deeper into the material.
The references provide theme and topic related literature, which you can read to learn new things and get new perspectives.
The related discussions tab shows every related discourse at iversity.org to the topic of the unit.