Do you mean a multi-level TOC or are you referring to something else?
If you do mean multi-level TOC then you could look at:
2 Level TOC:
https://www.mobileread.com/forums/sho...d.php?t=219302
3 Level TOC:
https://www.mobileread.com/forums/sho...d.php?t=134894
On my Sony PRS-300 the levels certainly display in the manner you describe. Presumably more levels could easily be defined but I don't know how many levels various readers support.