Sectioning content is identified as defining "the scope of headings and footers" [1].

Both <footer> [2] and <header> [3] elements are scoped to their nearest ancestor sectioning content or sectioning root element. As such, sectioning content defines the scope of headers as much as footers, and this should be reflected in the definition of sectioning content, e.g. "Sectioning content is content that defines the scope of headings, headers and footers."

[1] http://www.w3.org/TR/html51/dom.html#sectioning-content-2
[2] http://www.w3.org/TR/html51/semantics.html#the-footer-element
[3] http://www.w3.org/TR/html51/semantics.html#the-header-element