A Section is used to identify a date range within a group. Members are then added to the section.

For example: 

Each group will have a section for the current year. Next year, there will be a change to the members included in that group. Therefore, a new section would created for the following year, rather than creating a new group. 

The same can be said for classes. A section is used to indicate not only the semester/quarter, but the teacher/professor/administrator as well.