For example, a computer-based book of art with minimal text, or a set of photographs or scans of pages, would not usually be called an "e-text".
An e-text may be a binary or a plain text file, viewed with any open source or proprietary software.
E-texts, or electronic documents, have been around since long before the Internet, the Web, and specialized E-book reading hardware.
[1] In some communities, "e-text" is used much more narrowly, to refer to electronic documents that are, so to speak, "plain vanilla ASCII".
Chapter and sections titles, likewise, are just additional lines of text: they might be detectable by capitalization if they were all caps in the original (or not).
A program cannot reliably tell where footnotes, headers or footers are, or perhaps even paragraphs, so it cannot re-arrange the text, for example to fit a narrower screen, or read it aloud for the visually impaired.
The main difference from more formal markup is that "plain texts" use implicit, usually undocumented conventions, which are therefore inconsistent and difficult to recognize.
For many years Project Gutenberg strongly favored this model of text, but with time, has begun to develop and distribute more capable forms such as HTML.