There are inline tocs, and toc files (EPUBs are just ZIP files with a different name and a file which defines where all the other files are in the zip). ToC files are used by readers when the user asks for the ToC (e.g. right clicking in calibre and clicking on Table of Contents). Whereas inline is like in printed book, where it will show up on one of the pages.
I haven’t used ABBYY so I don’t know how to do it in that. But Calibre (which I do use) has an easy “Insert inline ToC”, which will take the toc file and insert it as a list
Norwegian