In some cases, teachers have very large courses with many learners, instructors, and teaching assistants, and use sections to split the course into manageable lists.
To filter by section, you must have permissions and access to view the sections in the course. If there are sections in a classlist, the students are automatically filtered into their respective section. Teachers can only filter on sections to which they have access permissions. If you have access to one section only, that section is automatically selected by default. Teachers with access permissions to multiple sections can remove the section filter by selecting All Users.