Keith:  1) the original TOC has no true menaing in an

     digital version.

The job of a TOC is to tell the reader the major subparts of the whole that the reader is reading, and how to get to one of those subparts if the reader doesn’t want to read the whole.  This has the same meaning in a digital books as in a paper book.

2) a user define TOC must be identifiable as just that.

     it actually does not matter where it goes! Yet, see 3

Today a TOC is frequently recognized as being the TOC by including a tag on it which says “TOC”

b) the format has generally a requirement for its placement.

Not aware of real ebook formats that place a requirement on the placement of at TOC. “real ebook formats” meaning in practice EPUB and MOBI.  People including most of PG are confused by TOC and EPUB because the official EPUB documentation is horribly written and confusing, referring to two distinct things as being TOCs, and by the fact that Adobe in their implementation of EPUB chose to ignore one of these two “TOC”s covered by the EPUB documentation, highlighting something as being a TOC which isn’t really a TOC but rather an “on the side” digital navigational aide. In MOBI a TOC is placed at location simply tagged as being a “TOC”

PS: re “Who am I quoting” – if he who I am quoting wanted to be part of a discussion then this nicety of distinction would seem to matter.