I figured it out. In case someone else needs to know:

The TOC level marked with an arrow in the illustration I attached corresponds, not to different TOCs, but to the Heading level within that TOC. TOC 2 was marked with an arrow in the illustration I attached because my cursor happened to be on a "Heading 2" entry in the existing TOC when I clicked on "Edit Field".

Clicking on the modify button allows you to modify the font and style for each heading entry. So, clicking on modify with TOC 1 selected modifies Heading 1, TOC 2 modifies Heading 2, etc.

I hope this helps someone else.