Nesting is how FictionBook expresses a hierarchy: sections inside sections, with no separate table of contents to keep in step.
The outer section carries prose of its own before the inner ones begin, which is legal and is where converters that assume leaf-only content lose text.
An inner section with a footnote reference[1] pointing into the notes body.
Three levels down. A generated table of contents should still reach this heading.
A cited block, which FB2 models as its own element rather than as a class on a paragraph.
The note itself lives in a second body element named notes - FictionBook's equivalent of an EPUB endnotes document marked linear="no".