The Add button increases the number of lines (sections).
The Remove button reduces the number of lines (sections).
The Set as default button makes the current section the default one. It is this section that will be initially selected when you select a test.
You can use the Section name column to edit the names of test sections.
You can use the Path to section column to edit the path to tests from this section. A double click on a line opens a special window where you can specify the path.