Each spreadsheet course consists of chapters, which consist of workbooks, which consist of exercises.
Course, chapters and exercises
Just like normal courses, a spreadsheet course has chapters and exercises. You can find the definitions of these concepts in the part about terminology and structure in the course design documentation.
The workbook is the bit where a spreadsheet course differentiates with a regular course. Each spreadsheet chapter is divided into one or more workbooks. You can think of it as a way to subdivide your chapters into chunks of exercises that are about the same dataset.
The division in workbooks will not be visible directly for the students, but it will be obvious from the authoring interface. It will help you as an author to group exercises that are about the same dataset together.