On the present website, the aggregation level or functional granularity of the learning materials is distinguished into the following four categories. By clicking on one of the categories, all materials that belong to that category are displayed. A clarification of each category is given after the link.

• Fragment: this is the smallest level of aggregation, mostly a part of a single lesson, e.g. a video fragment, an explanation of a role play, etc.

• Lesson: a lesson is a collection of level 1 learning objects, together answering the learning objective(s) and to be completed within a restricted amount of time (typically ½-1 h, but may endure up to 3-4 h including breaks).

• Module: a collection of level 2 learning objects together forming an educational unit and treating an isolated part of a broad topic, typically demanding for one half to several days.

• Course: a collection of related level 3 learning objects which may demand for some days, weeks, or even more.