Footnotes are bits of text tagged on at the end, like Wikipedia for instance. UBook semantically tags them so they are available where the note exists in the book. Presumably a popup or panel when you touch or click the note marker in the text. It was a bad use of words. It was meant to convey the idea you didn't jump to the end of the chapter or book to read a note. HTML-based formats just use anchors.

Yes, compiler is ambiguous. The idea behind it is a clean interim language a writing tool could export to, and software could then adapt to other formats like Kindle etc.