How can I create subsections in Table of Contents

I'm sure I've seen them in some of my books, but I couldn't find exactly what I was looking for for an example. The closest I've found was similar to below:

Front Matter(followed by a down arrow)
and then the normal TOC continues.

If you tap the down arrow, it changes to a right arrow and all the subsections are hidden. (I might have the direction of the arrows reversed, but...) :D

Looking at the code in Sigil, I couldn't see anything that would cause this behavior, and a search for "Front Matter" had no results.

I have a couple enormous books where I would like to break the ebook down into the same sections as the actual books, i.e.

Section OneSection TwoSection Three, etc.

Is this possible, and if so, how do you do it?
Thanks for your help.