To create sub sections in your bundle, you must first turn off automatic section numbering - this will allow you to manually name/number your sections to reflect your main and sub sections.
With automatic section numbering off, you can manually name/number your main and sub sections by double clicking the section in the "Sections" pane. In the example below, our main sections are numbered (eg, '1.', '2.', '3.', etc), and our sub section is numbered after the '.' (eg, '1.1', 1.2', '1.3'). This method of manually numbering/naming your sections and sub sections allows for any number of styles (eg 'A-1', 'A-1.1', 'B-1', 'B1.1').
To further differentiate your main sections from your sub sections in the index, we recommend adjusting the "Separator colour" of your sections in the section properties - this will change the background fill of the section heading in the index. To do this, open the section properties pane by right clicking the section and selecting properties:
With the properties pane open, click the coloured box in the "Separator colour" row. This will allow you to select a colour.
In this example, we elected to use a grey colour for our main sections and white for our sub sections. With all this applied, the index will look like this: