Bundles (short for the "bundle of learning") hold the learning delivery organization: units, sections, chapters, weeks, modules, etc. This kind of “topic” organization is typically the way the course (level-4 object) is organized in a syllabus, LMS, textbook, or courseware.
In Coursetune we call this layer “Bundles” until you designate what it really should be called for each course. In the example below, the bundles have been renamed to topics.
Want a better understanding of the two layers in Coursetune? We recommend this article: Why does Coursetune have two course (level-4 object) design layers?